ipv4: introduce __ip_dev_find()

ip_dev_find(net, addr) finds a device given an IPv4 source address and
takes a reference on it.

Introduce __ip_dev_find(), taking a third argument, to optionally take
the device reference. Callers not asking the reference to be taken
should be in an rcu_read_lock() protected section.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>