xfrm_user: validate XFRM_MSG_NEWAE incoming ESN size harder

xfrm_user: validate XFRM_MSG_NEWAE incoming ESN size harder

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.

CVE-2017-7184
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name xfrm_user-validate-xfrm_msg_newae-incoming-esn-size-harder.patch