btrfs: output human readable space info flag

For btrfs_space_info, its flags has only 4 possible values:

- BTRFS_BLOCK_GROUP_SYSTEM
- BTRFS_BLOCK_GROUP_METADATA | BTRFS_BLOCK_GROUP_DATA
- BTRFS_BLOCK_GROUP_METADATA
- BTRFS_BLOCK_GROUP_DATA

Make the output more human readable, now it looks like:

  BTRFS info (device dm-1: state A): space_info METADATA has 251494400 free, is not full
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -476,18 +476,35 @@ do {									\
 	spin_unlock(&__rsv->lock);					\
 } while (0)
 
+static const char *space_info_flag_to_str(const struct btrfs_space_info *space_info)
+{
+	switch (space_info->flags) {
+	case BTRFS_BLOCK_GROUP_SYSTEM:
+		return "SYSTEM";
+	case BTRFS_BLOCK_GROUP_METADATA | BTRFS_BLOCK_GROUP_DATA:
+		return "DATA+METADATA";
+	case BTRFS_BLOCK_GROUP_DATA:
+		return "DATA";
+	case BTRFS_BLOCK_GROUP_METADATA:
+		return "METADATA";
+	default:
+		return "UNKNOWN";
+	}
+}
+
 static void __btrfs_dump_space_info(struct btrfs_fs_info *fs_info,
 				    struct btrfs_space_info *info)
 {
+	const char *flag_str = space_info_flag_to_str(info);
 	lockdep_assert_held(&info->lock);
 
 	/* The free space could be negative in case of overcommit */
-	btrfs_info(fs_info, "space_info %llu has %lld free, is %sfull",
-		   info->flags,
+	btrfs_info(fs_info, "space_info %s has %lld free, is %sfull",
+		   flag_str,
 		   (s64)(info->total_bytes - btrfs_space_info_used(info, true)),
 		   info->full ? "" : "not ");
 	btrfs_info(fs_info,
-		"space_info total=%llu, used=%llu, pinned=%llu, reserved=%llu, may_use=%llu, readonly=%llu zone_unusable=%llu",
+"space_info total=%llu, used=%llu, pinned=%llu, reserved=%llu, may_use=%llu, readonly=%llu zone_unusable=%llu",
 		info->total_bytes, info->bytes_used, info->bytes_pinned,
 		info->bytes_reserved, info->bytes_may_use,
 		info->bytes_readonly, info->bytes_zone_unusable);