xfrm: fix sa selector validation

After commit b38ff40, the following command does not work anymore:
$ ip xfrm state add src 10.125.0.2 dst 10.125.0.1 proto esp spi 34 reqid 1 \
  mode tunnel enc 'cbc(aes)' 0xb0abdba8b782ad9d364ec81e3a7d82a1 auth-trunc \
  'hmac(sha1)' 0xe26609ebd00acb6a4d51fca13e49ea78a72c73e6 96 flag align4

In fact, the selector is not mandatory, allow the user to provide an empty
selector.

Fixes: b38ff40 ("xfrm: Fix xfrm sel prefix length validation")
CC: Anirudh Gupta <anirudh.gupta@sophos.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

@@ -166,6 +166,9 @@ static int verify_newsa_info(struct xfrm_usersa_info *p,
 	}
 
 	switch (p->sel.family) {
+	case AF_UNSPEC:
+		break;
+
 	case AF_INET:
 		if (p->sel.prefixlen_d > 32 || p->sel.prefixlen_s > 32)
 			goto out;