[IPV4]: Small style cleanup of the error path in rtm_to_ifaddr.

Remove error code assignment inside brackets on failure. The code
looks better if the error is assigned before condition check. Also,
the compiler treats this better.

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