Virtualize if_edsc(4).
Change the UMA mutex into a rwlock

Acquire the lock in read mode when just needed to ensure the stability
of the keg list. The UMA lock may be held for a long time (relatively
speaking) in uma_reclaim() on machines with lots of zones/kegs. If the
uma_timeout() would fire during that period, subsequent callouts on that
CPU may be significantly delayed.

Reviewed by:	jhb
seq_t needs to be visible to userspace

Pointy hat to:	mjg
Reported by: bz
X-MFC:	with r272567