locks: consolidate NULL i_flctx checks in locks_remove_file

We have each of the locks_remove_* variants doing this individually.
Have the caller do it instead, and have locks_remove_flock and
locks_remove_lease just assume that it's a valid pointer.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>

diff --cc fs/locks.c
Simple merge