filelock: WARN_ON_ONCE when ->fl_file and filp don't match

filelock: WARN_ON_ONCE when ->fl_file and filp don't match

vfs_lock_file, vfs_test_lock and vfs_cancel_lock all take both a struct
file argument and a file_lock. The file_lock has a fl_file field in it
howevever and it _must_ match the file passed in.

While most of the locks.c routines use the separately-passed file
argument, some filesystems rely on fl_file being filled out correctly.

I'm working on a patch series to remove the redundant argument from
these routines, but for now, let's ensure that the callers always set
this properly by issuing a WARN_ON_ONCE if they ever don't match.

Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Trond Myklebust <trondmy@hammerspace.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>