filelock: always define for_each_file_lock()

filelock: always define for_each_file_lock()

...and eliminate the stub version when CONFIG_FILE_LOCKING is disabled.
This silences the following warning that crept in recently:

fs/ceph/locks.c: In function 'ceph_count_locks':
fs/ceph/locks.c:380:27: error: unused variable 'lock' [-Werror=unused-variable]
  380 |         struct file_lock *lock;

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402062210.3YyBVGF1-lkp@intel.com/
Fixes: 75cabec0111b ("filelock: add some new helper functions")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240212-flsplit3-v1-1-019f0ad6bf69@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>