Relax the locking requirements for vm_object_page_noreuse(). While
reviewing all uses of OFF_TO_IDX(), I observed that
vm_object_page_noreuse() is requiring an exclusive lock on the object
when, in fact, a shared lock suffices.

Reviewed by: kib, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D10011