net: ipv4: fix schedule while atomic bug in check_lifetime()

move might_sleep operations out of the rcu_read_lock() section.
Also fix iterating over ifa_dev->ifa_list

Introduced by: commit 5c766d64

 "ipv4: introduce address lifetime"

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>