Fix snapshot automount expiry cancellation deadlock

A deadlock occurs when snapshot expiry tasks are cancelled while holding
locks. The snapshot expiry task (snapentry_expire) spawns an umount
process and waits for it to complete. Concurrently, ARC memory pressure
triggers arc_prune which calls zfs_exit_fs(), attempting to cancel the
expiry task while holding locks. The umount process spawned by the
expiry task blocks trying to acquire locks held by arc_prune, which is
blocked waiting for the expiry task to complete. This creates a circular
dependency: expiry task waits for umount, umount waits for arc_prune,
arc_prune waits for expiry task.

Fix by adding non-blocking cancellation support to taskq_cancel_id().
The zfs_exit_fs() path calls zfsctl_snapshot_unmount_delay() to
reschedule the unmount, which needs to cancel any existing expiry task.
It now uses non-blocking cancellation to avoid waiting while holding
locks, breaking the deadlock by returning immediately when the task is
already running.

The per-entry se_taskqid_lock has been removed, with all taskqid
operations now protected by the global zfs_snapshot_lock held as
WRITER. Additionally, an se_in_umount flag prevents recursive waits when
zfsctl_destroy() is called during unmount. The taskqid is now only
cleared by the caller on successful cancellation; running tasks clear
their own taskqid upon completion.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17941

module/zfs/zfs_fm.c

@@ -1531,7 +1531,8 @@ zfs_ereport_taskq_fini(void)
 {
 	mutex_enter(&recent_events_lock);
 	if (recent_events_cleaner_tqid != 0) {
-		taskq_cancel_id(system_delay_taskq, recent_events_cleaner_tqid);
+		taskq_cancel_id(system_delay_taskq, recent_events_cleaner_tqid,
+		    B_TRUE);
 		recent_events_cleaner_tqid = 0;
 	}
 	mutex_exit(&recent_events_lock);