ping6: Fix data type of the buffer for ancillary data of a received message

The old code worked, but wasted some stack space.

Submitted by:	Ján Sučan <sucanjan@gmail.com>
MFC after:	2 weeks
Sponsored by:	Google, inc. (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21275

sbin/ping6/ping6.c

@@ -307,7 +307,7 @@ main(int argc, char *argv[])
 	int ip6optlen = 0;
 	struct cmsghdr *scmsgp = NULL;
 	/* For control (ancillary) data received from recvmsg() */
-	struct cmsghdr cm[CONTROLLEN];
+	u_char cm[CONTROLLEN];
 #if defined(SO_SNDBUF) && defined(SO_RCVBUF)
 	u_long lsockbufsize;
 	int sockbufsize = 0;