esp4: 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 |
    | IP hdr | ESP | opts.| TCP | Data | Trailer | ICV |
    ----------------------------------------------------

Before doing gso segment in xfrm4_beet_gso_segment(), the same
thing is needed as it does in xfrm6_beet_gso_segment() in last
patch 'esp6: support ipv6 nexthdrs process for 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: 384a46ea

 ("esp4: add gso_segment for esp4 beet mode")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>