locks: consolidate checks for compatible filp->f_mode values in setlk handlers

Move this check into flock64_to_posix_lock instead of duplicating it in
two places. This also fixes a minor wart in the code where we continue
referring to the struct flock after converting it to struct file_lock.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>