vm_page_xbusy_claim(): Use atomics to update busy lock state.

vm_page_xbusy_claim(): Use atomics to update busy lock state.

vm_page_xbusy_claim() could clobber the waiter bit.  For its original
use, kernel memory pages, this was not a problem since nothing would
ever block on the busy lock for such pages.  r363607 introduced a new
use where this could in principle be a problem.

Fix the problem by using atomic_cmpset to update the lock owner.  Since
this macro is defined only for INVARIANTS kernels the extra overhead
doesn't seem prohibitive.

Reported by:	vangyzen
Reviewed by:	alc, kib, vangyzen
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25859