Remove some redundant assignments and computations.

Alike to geom_disk free the provider statistics structure and point GEOM
toward local statistics.  It allows to save some CPU time.

Properly flush outstanding I/Os when forcibly deleteing a memory disk device.

Sponsored by: Netflix

On PowerNV systems, the rootfs is passed through kexec, which loads the rootfs
into memory and set two fdt entries to describe where the file is located in
the memory;

I need to pass this memory region to the md device as a mfs_root, but, current
md driver does not support two things:

 * Just getting a pointer from an external (bootloader) memory. If I need to
workaround it, I would need to declare a static array and memcopy from this
external memory to this static variable.

 * The size of the image. The usage of mfs_root_end, which is not a pointer,
seems to be not possible for this prestaged scenario.

This patch simply adds a new way to load mfs_root from memory.

Differential Revision: https://reviews.freebsd.org/D15625
Approved by: kib, jhibbits (mentor)

opt_compat.h is mentioned in nearly 180 files. In-progress network
driver compabibility improvements may add over 100 more so this is
closer to "just about everywhere" than "only some files" per the
guidance in sys/conf/options.

Keep COMPAT_LINUX32 in opt_compat.h as it is confined to a subset of
sys/compat/linux/*.c.  A fake _COMPAT_LINUX option ensure opt_compat.h
is created on all architectures.

Move COMPAT_LINUXKPI to opt_dontuse.h as it is only used to control the
set of compiled files.

Reviewed by:	kib, cem, jhb, jtl
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14941

This is more correct in that ioctl commands have no meaning until they
hit the handler associated with the file descriptor.

Add support for MDIOCRESIZE_32 which was missed when it was added.

Reviewed by:	cem, kib, markj (various versions)
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14714

Include _uio.h instead of uio.h in several headers to reduce header
polution.

Fix a few places that relied on header polution to get the uio.h header.

I have not moved struct uio as many more things that use it rely on
header polution to get other definitions from uio.h.

Reviewed by:	cem, kib, markj
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14811

This allows compatibility translation to take place on the stack
(md_ioctl is too big) and is more suitable as a public interface within
the kernel than the kern_ioctl interface.

Except for the initialization of the md_req from the md_ioctl
(including detection of kernel md_file pointers) and the updating
of the md_ioctl prior to return, this is a mechanical replacment
of md_ioctl and mdio with md_req and mdr.

Reviewed by:	markj, cem, kib (assorted versions)
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14704

Move locks from outside ioctl to the individual implementations.

This is the first step of changing the implementations to act on a
kernel-internal request struct rather than on struct md_ioctl and to
removing the use of kern_ioctl in mountroot.

Reviewed by:	cem, kib, markj (prior version)
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14700

unconditionally incrementing i in the loop;

Reported by:	cem
MFC with:	r330880
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14685