Add a small helper ptype_head() to get the head to manipulate

dev_add_pack() & __dev_remove_pack() can use a spinlock without
blocking BH, since softirq use RCU, and these functions are run from
process context only.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>