xfrm: Validate address prefix lengths in the xfrm selector.

We don't validate the address prefix lengths in the xfrm
selector we got from userspace. This can lead to undefined
behaviour in the address matching functions if the prefix
is too big for the given address family. Fix this by checking
the prefixes and refuse SA/policy insertation when a prefix
is invalid.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Air Icy <icytxw@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>