net: Clarify meaning of CHECKSUM_PARTIAL for receive path

The current meaning of CHECKSUM_PARTIAL for validating checksums
is that _all_ checksums in the packet are considered valid.
However, in the manner that CHECKSUM_PARTIAL is set only the checksum
at csum_start+csum_offset and any preceding checksums may
be considered valid. If there are checksums in the packet after
csum_offset it is possible they have not been verfied.

This patch changes CHECKSUM_PARTIAL logic in skb_csum_unnecessary and
__skb_gro_checksum_validate_needed to only considered checksums
referring to csum_start and any preceding checksums (with starting
offset before csum_start) to be verified.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>