the destroying cdev.

Currently linux_destroy_dev() waits for the reference count on the
linux cdev to drain, and each open file hold the reference.
Practically it means that linux_destroy_dev() is blocked until all
userspace processes that have the cdev open, exit.  FreeBSD devfs does
not have such problem, because device refcount only prevents freeing
of the cdev memory, and separate 'active methods' counter blocks
destroy_dev() until all threads leave the cdevsw methods.  After that,
attempts to enter cdevsw methods are refused with an error.

Implement somewhat similar mechanism for LinuxKPI cdevs.  Demote cdev
refcount to only mean a hold on the linux cdev memory.  Add sirefs
count to track both number of threads inside the cdev methods, and for
single-bit indicator that cdev is being destroyed.  In the later case,
the call is redirected to the dummy cdev.

Reviewed by:	markj
Discussed with:	hselasky
Tested by:	zeising
MFC after:	1 week
Sponsored by:	Mellanox Technologies
Differential revision:	https://reviews.freebsd.org/D18606