Btrfs: fix memory leak in btrfs_create_tree()

We should free leaf and root before returning from the error
handling code.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>