xfrm: ipcomp: fix truesize computation on receive

ipcomp_post_acomp currently drops all frags (via pskb_trim_unique(skb,
0)), and then subtracts the old skb->data_len from truesize. This
adjustment has already be done during trimming (in skb_condense), so
we don't need to do it again.

This shows up for example when running fragmented traffic over ipcomp,
we end up hitting the WARN_ON_ONCE in skb_try_coalesce.

Fixes: eb2953d2 ("xfrm: ipcomp: Use crypto_acomp interface")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>