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

commit 4b654acd upstream.

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>
[PG: adapt for different file name in older code base.]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>