ipv6: sr: update the struct ipv6_sr_hdr

ipv6: sr: update the struct ipv6_sr_hdr

The IPv6 Segment Routing Header (SRH) format has been updated (revision 6
of the SRH ietf draft). The update includes the following SRH fields:

(1) The "First Segment" field changed to be "Last Entry" which contains
the index, in the Segment List, of the last element of the Segment List.

(2) The 16 bit "reserved" field now is used as a "tag" which tags a packet
as part of a class or group of packets, e.g.,packets sharing the same
set of properties.

This patch updates the struct ipv6_sr_hdr, so it complies with the updated
SRH draft. The 16 bit "reserved" field is changed to be "tag", In addition
a comment is added to the "first_segment" field, showing that it represents
the "Last Entry" field of the SRH.

Signed-off-by: Ahmed Abdelsalam <amsalam20@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>