systrace.c fails to build if we're using a common compiler.h for both
openzfs and linuxkpi. The issue is easy enough to fix: don't redefined
lower_32_bits if it's already defined in linux.h, since it's the least
'standardized'. This will allow systrace.c to build using an equivalent
macro.

MFC After: 3 days
Sponsored by: Netflix

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

The two main uses of dev_t are in struct stat and as a parameter of the
mknod system calls.
As of version 2.6.0 of the Linux kernel, dev_t is a 32-bit quantity
with 12 bits set asaid for the major number and 20 for the minor number.
The in-kernel dev_t encoded as MMMmmmmm, where M is a hex digit of the
major number and m is a hex digit of the minor number.
The user-space dev_t encoded as mmmM MMmm, where M and m is the major
and minor numbers accordingly. This is downward compatible with legacy
systems where dev_t is 16 bits wide, encoded as MMmm.
In glibc dev_t is a 64-bit quantity, with 32-bit major and minor numbers,
encoded as MMMM Mmmm mmmM MMmm. This is downward compatible with the Linux
kernel and with legacy systems where dev_t is 16 bits wide.
In the FreeBSD dev_t is a 64-bit quantity. The major and minor numbers
are encoded as MMMmmmMm, therefore conversion of the device numbers between
Linux user-space and FreeBSD kernel required.

As of version 2.6.0 of the Linux kernel, dev_t is a 32-bit unsigned integer
on all platforms. Move it into the MI linux.h under /compat/linux.

Migrate linux and linprocfs to use the IfAPI interfaces instead of
direct ifnet accesses.
The code initially writed by jhibbits@, and adapted by me to 3ab3c9c2.

Reviewed by:		jhibbits
Differential Revision:  https://reviews.freebsd.org/D38735

Revert pending netlink fixes, and further fixes to this.

This reverts commit 52d984831d82d97dc132d0d57fca6ee89572799b.

Requested by:	dchagin

Reviewed by:	dchagin
Sponsored by:	Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38349

MI linux.[c|h] are the module independent in terms of the Linux emulation
layer (ie, intended for both ISA - 32 & 64 bit), analogue of MD linux.h.
There must be a code here that cannot be placed into the corresponding by
common sense MI source and header files, i.e., code is machine independent,
but ISA dependent.
For the use_real_names knob, the code must be placed into the
linux_socket.[c|h], however linux_socket is ISA dependent.

MFC after:		2 weeks

Leftover after converting futexes to the umtx API.

NFC after:		2 weeks