Btrfs: fix verification of dir_item

We were ignoring the name component of the dir_item. Both the
name and data must fit within BTRFS_MAX_XATTR_SIZE(root).

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>