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

module/zfs/spa.c

@@ -8173,6 +8173,7 @@ spa_scan_stop(spa_t *spa)
 	ASSERT(spa_config_held(spa, SCL_ALL, RW_WRITER) == 0);
 	if (dsl_scan_resilvering(spa->spa_dsl_pool))
 		return (SET_ERROR(EBUSY));
+
 	return (dsl_scan_cancel(spa->spa_dsl_pool));
 }
 
@@ -8198,6 +8199,10 @@ spa_scan(spa_t *spa, pool_scan_func_t func)
 		return (0);
 	}
 
+	if (func == POOL_SCAN_ERRORSCRUB &&
+	    !spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG))
+		return (SET_ERROR(ENOTSUP));
+
 	return (dsl_scan(spa->spa_dsl_pool, func));
 }
 
@@ -9249,6 +9254,7 @@ spa_sync_iterate_to_convergence(spa_t *spa, dmu_tx_t *tx)
 		brt_sync(spa, txg);
 		ddt_sync(spa, txg);
 		dsl_scan_sync(dp, tx);
+		dsl_errorscrub_sync(dp, tx);
 		svr_sync(spa, tx);
 		spa_sync_upgrades(spa, tx);