Split key_sp2mbuf() function into key_sp2mbuf() and key_sp2msg().

Make key_sp2mbuf() static for private use only in key.c.
key_sp2msg() takes buffer and its size as arguments. When given buffer
is not enough to store security policy, function calculates needed size
and returns it back via sadb_x_policy_len field of struct sadb_x_policy.
Also reflect changing of struct ipsecrequest.