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

man/man7/vdevprops.7

@@ -45,7 +45,7 @@ section, below.
 Every vdev has a set of properties that export statistics about the vdev
 as well as control various behaviors.
 Properties are not inherited from top-level vdevs, with the exception of
-checksum_n, checksum_t, io_n, io_t, slow_io_n, and slow_io_t.
+checksum_n, checksum_t, io_n, io_t, slow_io_events, slow_io_n, and slow_io_t.
 .Pp
 The values of numeric properties can be specified using human-readable suffixes
 .Po for example,
@@ -149,6 +149,12 @@ For
 .Sy OpenZFS on FreeBSD
 defaults see
 .Xr zfsd 8 .
+The
+.It Sy slow_io_events
+property controls whether slow I/O events are generated.
+Even when disabled, slow I/Os will be included in the
+.Nm zpool Cm status Fl s
+output.
 .It Sy comment
 A text comment up to 8192 characters long
 .It Sy bootsize