net/ipv4: correct RFC 1122 section reference in comment

RFC 1122 does not have a section 3.1.2.2. The requirement to silently
discard datagrams with a bad checksum is in section 3.2.1.2 instead.

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=10611

Signed-off-by: J.H.M. Dassen (Ray) <jdassen@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>