[IPV4]: Remove ifa != NULL check.

This is a callback registered to inet address notifier chain.
The check is useless as:
- ifa is always != NULL
- similar checks are abscent in all other notifiers.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>