[IPSEC] esp: Defer output IV initialization to first use.

First of all, if the xfrm_state only gets used for input
packets this entropy is a complete waste.

Secondly, it is often the case that a configuration loads
many rules (perhaps even dynamically) and they don't all
necessarily ever get used.

This get_random_bytes() call was showing up in the profiles
for xfrm_state inserts which is how I noticed this.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/esp.h

@@ -15,13 +15,14 @@ struct esp_data
 	struct {
 		u8			*key;		/* Key */
 		int			key_len;	/* Key length */
-		u8			*ivec;		/* ivec buffer */
+		int			padlen;		/* 0..255 */
 		/* ivlen is offset from enc_data, where encrypted data start.
 		 * It is logically different of crypto_tfm_alg_ivsize(tfm).
 		 * We assume that it is either zero (no ivec), or
 		 * >= crypto_tfm_alg_ivsize(tfm). */
 		int			ivlen;
-		int			padlen;		/* 0..255 */
+		int			ivinitted;
+		u8			*ivec;		/* ivec buffer */
 		struct crypto_blkcipher	*tfm;		/* crypto handle */
 	} conf;