LinuxKPI: Factor out vmf_insert_pfn_prot() routine

from GEM and TTM page fault handlers and move it in to base system. This
code is tightly integrated with LKPI mmap support to belong to drm-kmod.

As this routine requires associated vm_object to be locked, it got
additional _locked suffix.

Reviewed by: hselasky, markj
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D32068