tcp: clear sendfile logging struct

The sendfile black box logging struct is much smaller than the
encompassing stack specific logging union. Be sure to clear the
trailing unused memory when logging.

Reviewed by:	tuexen
Sponsored by:	Netflix, Inc.

(cherry picked from commit 3bd1e85fc1)

sys/netinet/tcp_log_buf.c

@@ -2878,6 +2878,7 @@ tcp_log_sendfile(struct socket *so, off_t offset, size_t nbytes, int flags)
 		struct timeval tv;
 		tcp_log_eventspecific_t log;
 
+		memset(&log, 0, sizeof(log));
 		microuptime(&tv);
 		log.u_sf.offset = offset;
 		log.u_sf.length = nbytes;