[IPV4]: Add ip_local_out

Most callers of the LOCAL_OUT chain will set the IP packet length and
header checksum before doing so.  They also share the same output
function dst_output.

This patch creates a new function called ip_local_out which does all
of that and converts the appropriate users over to it.

Apart from removing duplicate code, it will also help in merging the
IPsec output path once the same thing is done for IPv6.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>