LinuxKPI: Allow cdev_pager prefault handler to steal pages

from other vm_objects. This workarounds "Page already inserted" panic
in vm_page_insert routine triggered on attempt to mmap file created
with shmem_file_setup call. After introduction of "GTT mmap
interface v4" a.k.a. MMAP_OFFSET, vm_objects allocated by these calls
may try to own intersected sets of pages that leads to the assertion.

Reviewed by: kib
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D32090