geom: Make g_waitidle() wait for orphaned providersHEADmain

This is motivated by the following race in the ZFS zvol code.

When a zvol is created, we create a GEOM-backed zvol, which results in a
/dev/zvol/<zvol path> device file, created by GEOM::dev.  If volmode=dev
is specified, zvol_set_volmode_impl() will wither the GEOM, then create
a device file with the same name.  This sometimes fails because
g_wither_geom() is asynchronous, so we end up trying to create a device
file while the old one still exists.  I want to fix this by adding a
g_waitidle() call to zvol_os_remove_minor().

g_waitidle() is not sufficient: GEOM::dev does not destroy the device
until g_dev_orphan() is called.  (In fact the device destruction is
asynchronous too, but the delist_dev() call is sufficient to address
this race.)  So, I propose modifying g_waitidle() to block until
orphaned providers are processed.

PR:		258766
Reviewed by:	mav, imp, kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D55049

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