btrfs: block-group: Fix a memory leak due to missing btrfs_put_block_group()

In btrfs_read_block_groups(), if we have an invalid block group which
has mixed type (DATA|METADATA) while the fs doesn't have MIXED_GROUPS
feature, we error out without freeing the block group cache.

This patch will add the missing btrfs_put_block_group() to prevent
memory leak.

Note for stable backports: the file to patch in versions <= 5.3 is
fs/btrfs/extent-tree.c

Fixes: 49303381 ("Btrfs: bail out if block group has different mixed flag")
CC: stable@vger.kernel.org # 4.9+
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
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/block-group.c

@@ -1761,6 +1761,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
 			btrfs_err(info,
 "bg %llu is a mixed block group but filesystem hasn't enabled mixed block groups",
 				  cache->key.objectid);
+			btrfs_put_block_group(cache);
 			ret = -EINVAL;
 			goto error;
 		}