ipv4: fix ineffective source address selection

When sending out multicast messages, the source address in inet->mc_addr is
ignored and rewritten by an autoselected one. This is caused by a typo in
commit 813b3b5d

 ("ipv4: Use caller's on-stack flowi as-is in output
route lookups").

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>