Make user supplied data checks a bit stricter.

key_msg2sp() is used for parsing data from setsockopt(IP[V6]_IPSEC_POLICY)
call. This socket option is usually used to configure IPsec bypass for
socket. Only privileged user can set this socket option.
The message syntax is described here
	http://www.kame.net/newsletter/20021210/

and our libipsec is usually used to create the correct request.
Add additional checks:
* that sadb_x_ipsecrequest_len is not out of bounds of user supplied buffer
* that src/dst's sa_len is the same
* that 2*sa_len is not out of bounds of user supplied buffer
* that 2*sa_len fits into bounds of sadb_x_ipsecrequest

Reported by:	Ilja van Sprundel