esp6: support ipv6 nexthdrs process for beet gso segment

For beet mode, when it's ipv6 inner address with nexthdrs set,
the packet format might be:

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

Before doing gso segment in xfrm6_beet_gso_segment(), it should
skip all nexthdrs and get the real transport proto, and set
transport_header properly.

This patch is to fix it by simply calling ipv6_skip_exthdr()
in xfrm6_beet_gso_segment().

v1->v2:
  - remove skb_transport_offset(), as it will always return 0
    in xfrm6_beet_gso_segment(), thank Sabrina's check.

Fixes: 7f9e40eb

 ("esp6: add gso_segment for esp6 beet mode")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>