xfrm: use memcpy to suppress gcc phony buffer overflow warning.

This bug is reported from below link:
https://bugzilla.redhat.com/show_bug.cgi?id=982761

An simplified command from its original reproducing method in bugzilla:
ip xfrm state add src 10.0.0.2 dst 10.0.0.1 proto ah spi 0x12345678 auth md5 12
will cause below spew from gcc.
Reported-by: Sohny Thomas <sthomas@linux.vnet.ibm.com>

ip/xfrm_state.c

@@ -162,7 +162,7 @@ static int xfrm_algo_parse(struct xfrm_algo *alg, enum xfrm_attr_type_t type,
 			if (len > max)
 				invarg("ALGO-KEYMAT value makes buffer overflow\n", key);
 
-			strncpy(buf, key, len);
+			memcpy(buf, key, len);
 		}
 	}