btrfs: sysfs: fix unused-but-set-variable warnings

The compilation with W=1 generates the following warnings:
 fs/btrfs/sysfs.c:1630:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  1630 |  int ret;
       |      ^~~
 fs/btrfs/sysfs.c:1629:6: warning: variable 'features' set but not used [-Wunused-but-set-variable]
  1629 |  u64 features;
       |      ^~~~~~~~

[ The unused variables are leftover from e410e34f ("Revert "btrfs:
  synchronize incompat feature bits with sysfs files""), which needs
  to be properly fixed by moving feature bit manipulation from the sysfs
  context.  Silence the warning to save pepople time, we got several
  reports. ]
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -1630,12 +1630,16 @@ void btrfs_sysfs_feature_update(struct btrfs_fs_info *fs_info,
 {
 	struct btrfs_fs_devices *fs_devs;
 	struct kobject *fsid_kobj;
-	u64 features;
-	int ret;
+	u64 __maybe_unused features;
+	int __maybe_unused ret;
 
 	if (!fs_info)
 		return;
 
+	/*
+	 * See 14e46e04958df74 and e410e34fad913dd, feature bit updates are not
+	 * safe when called from some contexts (eg. balance)
+	 */
 	features = get_features(fs_info, set);
 	ASSERT(bit & supported_feature_masks[set]);