[IPSEC] pfkey: Load specific algorithm in pfkey_add rather than all

This is a natural extension of the changeset

    [XFRM]: Probe selected algorithm only.

which only removed the probe call for xfrm_user.  This patch does exactly
the same thing for af_key.  In other words, we load the algorithm requested
by the user rather than everything when adding xfrm states in af_key.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>