ipv4: Fix 'iph' use before set.

I swear none of my compilers warned about this, yet it is so
obvious.

> net/ipv4/ip_forward.c: In function 'ip_forward':
> net/ipv4/ip_forward.c:87: warning: 'iph' may be used uninitialized in this function

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>