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. Since this macro is defined
only for INVARIANTS kernels, the extra overhead doesn't seem
Reported by: vangyzen