btrfs: Fix memory leak in btrfs_read_fs_root_no_radix()

In btrfs_read_fs_root_no_radix(), 'root' is not freed if
btrfs_search_slot() returns error.

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