Teach zpool scrub to scrub only blocks in error log

Added a flag '-e' in zpool scrub to scrub only blocks in error log. A
user can pause, resume and cancel the error scrub by passing additional
command line arguments -p -s just like a regular scrub. This involves
adding a new flag, creating new libzfs interfaces, a new ioctl, and the
actual iteration and read-issuing logic. Error scrubbing is executed in
multiple txg to make sure pool performance is not affected.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Co-authored-by: TulsiJain tulsi.jain@delphix.com
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #8995
Closes #12355

man/man4/zfs.4

@@ -1764,6 +1764,9 @@ Scrubs are processed by the sync thread.
 While scrubbing, it will spend at least this much time
 working on a scrub between TXG flushes.
 .
+.It Sy zfs_scrub_error_blocks_per_txg Ns = Ns Sy 4096 Pq uint
+Error blocks to be scrubbed in one txg.
+.
 .It Sy zfs_scan_checkpoint_intval Ns = Ns Sy 7200 Ns s Po 2 hour Pc Pq uint
 To preserve progress across reboots, the sequential scan algorithm periodically
 needs to stop metadata scanning and issue all the verification I/O to disk.

man/man8/zpool-scrub.8

@@ -38,6 +38,7 @@
 .Cm scrub
 .Op Fl s Ns | Ns Fl p
 .Op Fl w
+.Op Fl e
 .Ar pool Ns …
 .
 .Sh DESCRIPTION
@@ -62,6 +63,13 @@ device
 whereas scrubbing examines all data to discover silent errors due to hardware
 faults or disk failure.
 .Pp
+When scrubbing a pool with encrypted filesystems the keys do not need to be
+loaded.
+However, if the keys are not loaded and an unrepairable checksum error is
+detected the file name cannot be included in the
+.Nm zpool Cm status Fl v
+verbose error report.
+.Pp
 Because scrubbing and resilvering are I/O-intensive operations, ZFS only allows
 one at a time.
 .Pp
@@ -92,9 +100,20 @@ Once resumed the scrub will pick up from the place where it was last
 checkpointed to disk.
 To resume a paused scrub issue
 .Nm zpool Cm scrub
+or
+.Nm zpool Cm scrub
+.Fl e
 again.
 .It Fl w
 Wait until scrub has completed before returning.
+.It Fl e
+Only scrub files with known data errors as reported by
+.Nm zpool Cm status Fl v .
+The pool must have been scrubbed at least once with the
+.Sy head_errlog
+feature enabled to use this option.
+Error scrubbing cannot be run simultaneously with regular scrubbing or
+resilvering, nor can it be run when a regular scrub is paused.
 .El
 .Sh EXAMPLES
 .Ss Example 1