xfrm_user: validate XFRM_MSG_NEWAE incoming ESN size harder

xfrm_user: validate XFRM_MSG_NEWAE incoming ESN size harder

Orabug: 25805996
CVE: CVE-2017-7184

Kees Cook has pointed out that xfrm_replay_state_esn_len() is subject to
wrapping issues.  To ensure we are correctly ensuring that the two ESN
structures are the same size compare both the overall size as reported
by xfrm_replay_state_esn_len() and the internal length are the same.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit f843ee6dd019bcece3e74e76ad9df0155655d0df)
Signed-off-by: Brian Maly <brian.maly@oracle.com>