xfrm: introduce oseq-may-wrap flag

RFC 4303 in section 3.3.3 suggests to disable anti-replay for manually
distributed ICVs in which case the sender does not need to monitor or
reset the counter. However, the sender still increments the counter and
when it reaches the maximum value, the counter rolls over back to zero.

This patch introduces new extra_flag XFRM_SA_XFLAG_OSEQ_MAY_WRAP which
allows sequence number to cycle in outbound packets if set. This flag is
used only in legacy and bmp code, because esn should not be negotiated
if anti-replay is disabled (see note in 3.3.3 section).

Signed-off-by: Petr Vaněk <pv@excello.cz>
Acked-by: Christophe Gouault <christophe.gouault@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>