vm_fault: update pred lock acq in copy_entry

In vm_fault_copy_entry, variable 'mpred' is invalided as the
predecessor of the next page to be inserted when an allocation failure
leads to lock release and reacquisition. Recompute it in that case.

Reported by: markj
Reviewed by: markj
Fixes: vm_page: expose page_alloc_after 6b33d9dc46c2f1cbb9127377119
Differential Revision: https://reviews.freebsd.org/D49521