LinuxKPI: Implement get_file_rcu()
get_file_rcu() grabs a file if the file->f_count is not zero.
Required by drm-kmod 5.6
Reviewed by: hselasky, manu (previous version)
Differential revision: https://reviews.freebsd.org/D31672
(cherry picked from commit a81b36c6d35d74177891860b789dd02b9d1c5851)