net: ipv6: drop duplicate word in comment

Drop the doubled word "by" in a comment.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/ipv6.h

@@ -223,7 +223,7 @@ struct ipv6_pinfo {
 
 	/*
 	 * Packed in 16bits.
-	 * Omit one shift by by putting the signed field at MSB.
+	 * Omit one shift by putting the signed field at MSB.
 	 */
 #if defined(__BIG_ENDIAN_BITFIELD)
 	__s16			hop_limit:9;