Btrfs: fix possible memory leak in btrfs_create_tree()

In btrfs_create_tree(), if btrfs_insert_root() fails, we should
free root->commit_root.

Reported-by: Alex Lyakas <alex@zadarastorage.com>
Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>