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: 75cabec ("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>

Author: jtlayton

include/linux/filelock.h

@@ -180,9 +180,6 @@ static inline void locks_wake_up(struct file_lock *fl)
 	wake_up(&fl->c.flc_wait);
 }
 
-/* for walking lists of file_locks linked by fl_list */
-#define for_each_file_lock(_fl, _head)	list_for_each_entry(_fl, _head, c.flc_list)
-
 /* fs/locks.c */
 void locks_free_lock_context(struct inode *inode);
 void locks_free_lock(struct file_lock *fl);
@@ -283,8 +280,6 @@ static inline void locks_wake_up(struct file_lock *fl)
 {
 }
 
-#define for_each_file_lock(_fl, _head)	while(false)
-
 static inline void
 locks_free_lock_context(struct inode *inode)
 {
@@ -414,6 +409,9 @@ locks_inode_context(const struct inode *inode)
 
 #endif /* !CONFIG_FILE_LOCKING */
 
+/* for walking lists of file_locks linked by fl_list */
+#define for_each_file_lock(_fl, _head)	list_for_each_entry(_fl, _head, c.flc_list)
+
 static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
 {
 	return locks_lock_inode_wait(file_inode(filp), fl);