datapath: 16bit inner_network_header field in struct ovs_gso_cb

The motivation for this is to create a 16bit hole in struct ovs_gso_cb
which may be used for the inner_protocol field which is needed
for the proposed implementation of compatibility for MPLS GSO segmentation.

This should be safe as inner_network_header is now an offset to
the inner_mac_header rather than skb->head.

As pointed out by Thomas Graf simply making both inner offsets 16bis is not
safe as there have been cases of overflow with "with collapsed TCP frames
on IB when the headroom grew beyond 64K. See commit 50bceae9bd ``tcp:
Reallocate headroom if it would overflow csum_start'' for additional
details."

This patch is based on suggestions by Thomas Graf and Jesse Gross.

Cc: Thomas Graf <tgraf@suug.ch>
Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>