net/xfrm_user: use in_compat_syscall to deny compat syscalls

The code wants to prevent compat code from receiving messages.  Use
in_compat_syscall for this.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>