libprocstat kstack: fix race with thread creation

When collecting kernel stacks for a target process, if the process
adds a thread between the two calls to sysctl, ignore the additional
threads.  Previously, procstat would print only a useless error
message.  Now, it prints a consistent snapshot of the stacks.
We know that snapshot is already stale, but it could still be stale
even with a more complex fix to reallocate and retry, so such a fix
is hardly worth the effort.

Reported by:	[email protected]
MFC after:	1 week
Sponsored by:	Dell EMC Isilon

(cherry picked from commit 427f12f)

lib/libprocstat/libprocstat.c

@@ -2610,7 +2610,8 @@ procstat_getkstack_sysctl(pid_t pid, int *cntp)
 		warn("malloc(%zu)", len);
 		return (NULL);
 	}
-	if (sysctl(name, nitems(name), kkstp, &len, NULL, 0) == -1) {
+	if (sysctl(name, nitems(name), kkstp, &len, NULL, 0) == -1 &&
+	    errno != ENOMEM) {
 		warn("sysctl: kern.proc.pid: %d", pid);
 		free(kkstp);
 		return (NULL);