[IPV4]: Fix setting broadcast for SIOCSIFNETMASK

Fix setting of the broadcast address when the netmask is set via
SIOCSIFNETMASK in Linux 2.6.  The code wanted the old value of
ifa->ifa_mask but used it after it had already been overwritten with
the new value.

Signed-off-by: David Engel <gigem@comcast.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>