xfrm: replay: remove advance indirection

Similar to other patches: add a new helper to avoid
an indirection.

v2: fix 'net/xfrm/xfrm_replay.c:519:13: warning: 'seq' may be used
uninitialized in this function' warning.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>