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
Differential revision: https://reviews.freebsd.org/D32068

(cherry picked from commit f6823dac8fa6e9fc2926a866e9a0c4d43e38e236)