Verify identity after checking for WAITFAIL in vm_page_busy_acquire().

A caller that does not guarantee that a page's identity won't change
while sleeping for a busy lock must specify either NOWAIT or WAITFAIL.

Reported by: syzkaller
Reviewed by: alc, kib
Discussed with: jeff
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22124