Btrfs: fix memory leak of chunks' extent map

commit 7d3d1744f8a7d62e4875bd69cc2192a939813880 upstream.

As we're hold a ref on looking up the extent map, we need to drop the ref
before returning to callers.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>