zed: protect against wait4()/fork() races to the global PID table

This can be very easily triggered by adding a sleep(1) before
the wait4() on a PID-starved system: the reaper thread would wait
for a child before its entry appeared, letting old entries accumulate:

  Invoking "all-debug.sh" eid=3021 pid=391
  Finished "(null)" eid=0 pid=391 time=0.002432s exit=0
  Invoking "all-syslog.sh" eid=3021 pid=336
  Finished "(null)" eid=0 pid=336 time=0.002432s exit=0
  Invoking "history_event-zfs-list-cacher.sh" eid=3021 pid=347
  Invoking "all-debug.sh" eid=3022 pid=349
  Finished "history_event-zfs-list-cacher.sh" eid=3021 pid=347
                                              time=0.001669s exit=0
  Finished "(null)" eid=0 pid=349 time=0.002404s exit=0
  Invoking "all-syslog.sh" eid=3022 pid=370
  Finished "(null)" eid=0 pid=370 time=0.002427s exit=0
  Invoking "history_event-zfs-list-cacher.sh" eid=3022 pid=391
  avl_find(tree, new_node, &where) == NULL
  ASSERT at ../../module/avl/avl.c:641:avl_add()
  Thread 1 "zed" received signal SIGABRT, Aborted.

By employing this wider lock, we atomise [wait, remove] and [fork, add]:
slowing down the reaper thread now just causes some zombies
to accumulate until it can get to them

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#11963
Closes openzfs#11965

cmd/zed/zed_exec.c

@@ -142,8 +142,10 @@ _zed_exec_fork_child(uint64_t eid, const char *dir, const char *prog,
 		    prog, eid, strerror(ENAMETOOLONG));
 		return;
 	}
+	(void) pthread_mutex_lock(&_launched_processes_lock);
 	pid = fork();
 	if (pid < 0) {
+		(void) pthread_mutex_unlock(&_launched_processes_lock);
 		zed_log_msg(LOG_WARNING,
 		    "Failed to fork \"%s\" for eid=%llu: %s",
 		    prog, eid, strerror(errno));
@@ -166,20 +168,19 @@ _zed_exec_fork_child(uint64_t eid, const char *dir, const char *prog,
 
 	/* parent process */
 
-	__atomic_sub_fetch(&_launched_processes_limit, 1, __ATOMIC_SEQ_CST);
-	zed_log_msg(LOG_INFO, "Invoking \"%s\" eid=%llu pid=%d",
-	    prog, eid, pid);
-
 	node = calloc(1, sizeof (*node));
 	if (node) {
 		node->pid = pid;
 		node->eid = eid;
 		node->name = strdup(prog);
 
-		(void) pthread_mutex_lock(&_launched_processes_lock);
 		avl_add(&_launched_processes, node);
-		(void) pthread_mutex_unlock(&_launched_processes_lock);
 	}
+	(void) pthread_mutex_unlock(&_launched_processes_lock);
+
+	__atomic_sub_fetch(&_launched_processes_limit, 1, __ATOMIC_SEQ_CST);
+	zed_log_msg(LOG_INFO, "Invoking \"%s\" eid=%llu pid=%d",
+	    prog, eid, pid);
 }
 
 static void