Ensure that the ERESTART return from the syscall reloads the
registers, to make the restarted syscall instruction pass the correct
arguments.

PR:	kern/182161
Reported by:	Russ Cox <rsc@swtch.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Approved by:	re (marius)
Use the pv lists generation count to read-lock the pvh_global_lock in
pmap_clear_modify().

Noted and reviewed by:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Approved by:	re (marius)