vm_fault: Shoot down shared mappings in vm_fault_copy_entry()

As in vm_fault_cow(), it's possible, albeit rare, for multiple vm_maps
to share a shadow object. When copying a page from a backing object
into the shadow, all mappings of the source page must therefore be
removed. Otherwise, future operations on the object tree may detect
that the source page is fully shadowed and thus can be freed.

Approved by: so
Security: FreeBSD-SA-22:11.vm
Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35635