xfrm: ipcomp: adjust transport header after decompressing

The skb transport header pointer needs to be adjusted by network header
pointer plus the size of the ipcomp header.

This shows up when running traffic over ipcomp using transport mode.
After being reinjected, packets are dropped because the header isn't
adjusted properly and some checks can be triggered. E.g the skb is
mistakenly considered as IP fragmented packet and later dropped.

kworker/30:1-mm 443 [030] 102.055250: skb:kfree_skb:skbaddr=0xffff8f104aa3ce00 rx_sk=(
ffffffff8419f1f4 sk_skb_reason_drop+0x94 ([kernel.kallsyms])
ffffffff8419f1f4 sk_skb_reason_drop+0x94 ([kernel.kallsyms])
ffffffff84281420 ip_defrag+0x4b0 ([kernel.kallsyms])
ffffffff8428006e ip_local_deliver+0x4e ([kernel.kallsyms])
ffffffff8432afb1 xfrm_trans_reinject+0xe1 ([kernel.kallsyms])
ffffffff83758230 process_one_work+0x190 ([kernel.kallsyms])
ffffffff83758f37 worker_thread+0x2d7 ([kernel.kallsyms])
ffffffff83761cc9 kthread+0xf9 ([kernel.kallsyms])
ffffffff836c3437 ret_from_fork+0x197 ([kernel.kallsyms])
ffffffff836718da ret_from_fork_asm+0x1a ([kernel.kallsyms])

Fixes: eb2953d26971 ("xfrm: ipcomp: Use crypto_acomp interface")
Link: https://bugzilla.suse.com/1244532
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>