btrfs: sysfs: replace direct access to feature set names with a helper

In order to unexport the feature type array, add a helper for the
enum-to-string conversion.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -5259,7 +5259,7 @@ static int check_feature_bits(struct btrfs_fs_info *fs_info,
 			      u64 change_mask, u64 flags, u64 supported_flags,
 			      u64 safe_set, u64 safe_clear)
 {
-	const char *type = btrfs_feature_set_names[set];
+	const char *type = btrfs_feature_set_name(set);
 	char *names;
 	u64 disallowed, unsupported;
 	u64 set_mask = flags & change_mask;

fs/btrfs/sysfs.c

@@ -675,12 +675,17 @@ void btrfs_sysfs_remove_mounted(struct btrfs_fs_info *fs_info)
 	btrfs_sysfs_rm_device_link(fs_info->fs_devices, NULL);
 }
 
-const char * const btrfs_feature_set_names[FEAT_MAX] = {
+static const char * const btrfs_feature_set_names[FEAT_MAX] = {
 	[FEAT_COMPAT]	 = "compat",
 	[FEAT_COMPAT_RO] = "compat_ro",
 	[FEAT_INCOMPAT]	 = "incompat",
 };
 
+const char * const btrfs_feature_set_name(enum btrfs_feature_set set)
+{
+	return btrfs_feature_set_names[set];
+}
+
 char *btrfs_printable_features(enum btrfs_feature_set set, u64 flags)
 {
 	size_t bufsize = 4096; /* safe max, 64 names * 64 bytes */

fs/btrfs/sysfs.h

@@ -78,7 +78,7 @@ attr_to_btrfs_feature_attr(struct attribute *attr)
 }
 
 char *btrfs_printable_features(enum btrfs_feature_set set, u64 flags);
-extern const char * const btrfs_feature_set_names[FEAT_MAX];
+const char * const btrfs_feature_set_name(enum btrfs_feature_set set);
 int btrfs_sysfs_add_device_link(struct btrfs_fs_devices *fs_devices,
 		struct btrfs_device *one_device);
 int btrfs_sysfs_rm_device_link(struct btrfs_fs_devices *fs_devices,