ipv4/GRO: Make GRO conform to RFC 6864

RFC 6864 states that the IPv4 ID field MUST NOT be used for purposes other
than fragmentation and reassembly.  Currently we are looking at this field
as a way of identifying what frames can be aggregated and  which cannot for
GRO.  While this is valid for frames that do not have DF set, it is invalid
to do so if the bit is set.

In addition we were generating IPv4 ID collisions when 2 or more flows were
interleaved over the same tunnel.  To prevent that we store the result of
all IP ID checks via a "|=" instead of overwriting previous values.

Change-Id: I96d90494f929fb217ceaa1698213465b0a17835a
CRs-Fixed: 2333588
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Patch-mainline: netdev @ April 1, 2016, 6:05 p.m
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: Manjunathappa Prakash <prakashpm@codeaurora.org>