btrfs: remove const from btrfs_feature_set_name

The function btrfs_feature_set_name returns a const char pointer, the
second const is not necessary and reported as a warning:

 In file included from fs/btrfs/space-info.c:6:
 fs/btrfs/sysfs.h:16:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
    16 | const char * const btrfs_feature_set_name(enum btrfs_feature_set set);
       | ^~~~~
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -973,7 +973,7 @@ static const char * const btrfs_feature_set_names[FEAT_MAX] = {
 	[FEAT_INCOMPAT]	 = "incompat",
 };
 
-const char * const btrfs_feature_set_name(enum btrfs_feature_set set)
+const char *btrfs_feature_set_name(enum btrfs_feature_set set)
 {
 	return btrfs_feature_set_names[set];
 }

fs/btrfs/sysfs.h

@@ -13,7 +13,7 @@ enum btrfs_feature_set {
 };
 
 char *btrfs_printable_features(enum btrfs_feature_set set, u64 flags);
-const char * const btrfs_feature_set_name(enum btrfs_feature_set set);
+const char *btrfs_feature_set_name(enum btrfs_feature_set set);
 int btrfs_sysfs_add_devices_dir(struct btrfs_fs_devices *fs_devices,
 		struct btrfs_device *one_device);
 int btrfs_sysfs_remove_devices_dir(struct btrfs_fs_devices *fs_devices,