esp6: get the right proto for transport mode in esp6_gso_encap

For transport mode, when ipv6 nexthdr is set, the packet format might
be like:

    ----------------------------------------------------
    |        | dest |     |     |      |  ESP    | ESP |
    | IP6 hdr| opts.| ESP | TCP | Data | Trailer | ICV |
    ----------------------------------------------------

What it wants to get for x-proto in esp6_gso_encap() is the proto that
will be set in ESP nexthdr. So it should skip all ipv6 nexthdrs and
get the real transport protocol. Othersize, the wrong proto number
will be set into ESP nexthdr.

This patch is to skip all ipv6 nexthdrs by calling ipv6_skip_exthdr()
in esp6_gso_encap().

Fixes: 7862b405 ("esp: Add gso handlers for esp4 and esp6")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>