kinfo_getfile: Add a comment explaining 4/3 ratio

We first get the expected size of the data from the kern.proc.filedesc
sysctl, allocate 4/3 of that size, and then fetch the data.  Add a
comment about the reason (the fd table may grow between the two calls).

Sponsored by:	The FreeBSD Foundation

lib/libutil/kinfo_getfile.c

@@ -27,6 +27,10 @@ kinfo_getfile(pid_t pid, int *cntp)
 	error = sysctl(mib, nitems(mib), NULL, &len, NULL, 0);
 	if (error)
 		return (NULL);
+	/*
+	 * Add extra space as the table may grow between requesting the size
+	 * and fetching the data.
+	 */
 	len = len * 4 / 3;
 	buf = malloc(len);
 	if (buf == NULL)