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

lib/libzfs/libzfs_util.c

@@ -1704,7 +1704,9 @@ zprop_parse_value(libzfs_handle_t *hdl, nvpair_t *elem, int prop,
 		    (prop == VDEV_PROP_CHECKSUM_N ||
 		    prop == VDEV_PROP_CHECKSUM_T ||
 		    prop == VDEV_PROP_IO_N ||
-		    prop == VDEV_PROP_IO_T)) {
+		    prop == VDEV_PROP_IO_T ||
+		    prop == VDEV_PROP_SLOW_IO_N ||
+		    prop == VDEV_PROP_SLOW_IO_T)) {
 			*ivalp = UINT64_MAX;
 		}