The vm_page_trysbusy() should not fail when shared busy counter or
VPB_BIT_WAITERS flag were changed between reading of busy_lock and the
cas. The vm_page_sbusy(), which is the only user of
vm_page_trysbusy() in the tree, panics on the failure, which in these
cases is transient and do not mean that the current page state
prevents sbusying.

Retry the operation inside vm_page_trysbusy() if cas failed, only
return a failure when VPB_BIT_SHARED is cleared.

Reported and tested by: pho
Reviewed by: attilio
Sponsored by: The FreeBSD Foundation