Btrfs: return error to caller when btrfs_unlink() failes

When btrfs_unlink_inode() and btrfs_orphan_add() in btrfs_unlink()
are error, the error code is returned to the caller instead of
BUG_ON().

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