xfrm: refactor XFRMA_MTIMER_THRESH validation into a helper

Extract verify_mtimer_thresh() to consolidate the XFRMA_MTIMER_THRESH
validation logic shared between the add_sa and upcoming patch.

Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>