netfilter: nf_ct_sip: don't drop packets with offsets pointing outside the packet

commit 3a7b21eaf4fb3c971bdb47a98f570550ddfe4471 upstream.

Some Cisco phones create huge messages that are spread over multiple packets.
After calculating the offset of the SIP body, it is validated to be within
the packet and the packet is dropped otherwise. This breaks operation of
these phones. Since connection tracking is supposed to be passive, just let
those packets pass unmodified and untracked.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: William Roberts <bill.c.roberts@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>