xfrm: ipcomp: Call pskb_may_pull in ipcomp_input

If a malformed packet is received there may not be enough data
to pull.  This isn't a problem in practice because the caller
has already done xfrm_parse_spi which in effect does the same
thing.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>