Add knob to disable slow io notifications

Introduce a new vdev property `VDEV_PROP_SLOW_IO_REPORTING` that
allows users to disable notifications for slow devices.
This prevents ZED and/or ZFSD from degrading the pool due to slow
I/O.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <oshogbo@FreeBSD.org>
Closes 17477

include/sys/fs/zfs.h

@@ -387,6 +387,7 @@ typedef enum {
 	VDEV_PROP_SLOW_IOS,
 	VDEV_PROP_SIT_OUT,
 	VDEV_PROP_AUTOSIT,
+	VDEV_PROP_SLOW_IO_EVENTS,
 	VDEV_NUM_PROPS
 } vdev_prop_t;