Batch v_wire_count decrements in vm_hold_free_pages().

Atomic updates to v_wire_count are a significant source of contention, so
combine multiple updates into one in this easy case. Also remove an old
printf that gets executed if the page is shared-busied, which is a case
that will lead to a panic anyway.

Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D11791