Add slow disk diagnosis to ZED

Slow disk response times can be indicative of a failing drive. ZFS
currently tracks slow I/Os (slower than zio_slow_io_ms) and generates
events (ereport.fs.zfs.delay).  However, no action is taken by ZED,
like is done for checksum or I/O errors.  This change adds slow disk
diagnosis to ZED which is opt-in using new VDEV properties:
  VDEV_PROP_SLOW_IO_N
  VDEV_PROP_SLOW_IO_T

If multiple VDEVs in a pool are undergoing slow I/Os, then it skips
the zpool_vdev_degrade().

Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Rob Wing <rob.wing@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #15469

man/man7/vdevprops.7

@@ -44,7 +44,7 @@ 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, and io_t.
+checksum_n, checksum_t, io_n, io_t, slow_io_n, and slow_io_t.
 .Pp
 The values of numeric properties can be specified using human-readable suffixes
 .Po for example,
@@ -117,7 +117,7 @@ If this device is currently being removed from the pool
 .Pp
 The following native properties can be used to change the behavior of a vdev.
 .Bl -tag -width "allocating"
-.It Sy checksum_n , checksum_t , io_n , io_t
+.It Sy checksum_n , checksum_t , io_n , io_t , slow_io_n , slow_io_t
 Tune the fault management daemon by specifying checksum/io thresholds of <N>
 errors in <T> seconds, respectively.
 These properties can be set on leaf and top-level vdevs.

man/man7/zpoolconcepts.7

@@ -260,8 +260,8 @@ sufficient replicas exist to continue functioning.
 The underlying conditions are as follows:
 .Bl -bullet -compact
 .It
-The number of checksum errors exceeds acceptable levels and the device is
-degraded as an indication that something may be wrong.
+The number of checksum errors or slow I/Os exceeds acceptable levels and the
+device is degraded as an indication that something may be wrong.
 ZFS continues to use the device as necessary.
 .It
 The number of I/O errors exceeds acceptable levels.

man/man8/zinject.8

@@ -69,6 +69,7 @@ Force a vdev into the DEGRADED or FAULTED state.
 .Nm zinject
 .Fl d Ar vdev
 .Fl D Ar latency : Ns Ar lanes
+.Op Fl T Ar read|write
 .Ar pool
 .Xc
 Add an artificial delay to I/O requests on a particular