Fix logging in l2arc_rebuild()

In case the L2ARC rebuild was canceled, do not log to spa history
log as the pool may be in the process of being removed and a panic
may occur:

BUG: kernel NULL pointer dereference, address: 0000000000000018
RIP: 0010:spa_history_log_internal+0xb1/0x120 [zfs]
Call Trace:
 l2arc_rebuild+0x464/0x7c0 [zfs]
 l2arc_dev_rebuild_start+0x2d/0x130 [zfs]
 ? l2arc_rebuild+0x7c0/0x7c0 [zfs]
 thread_generic_wrapper+0x78/0xb0 [spl]
 kthread+0xfb/0x130
 ? IS_ERR+0x10/0x10 [spl]
 ? kthread_park+0x90/0x90
 ret_from_fork+0x35/0x40

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10659

module/zfs/arc.c

@@ -9692,6 +9692,13 @@ l2arc_rebuild(l2arc_dev_t *dev)
 		    "no valid log blocks");
 		bzero(l2dhdr, dev->l2ad_dev_hdr_asize);
 		l2arc_dev_hdr_update(dev);
+	} else if (err == ECANCELED) {
+		/*
+		 * In case the rebuild was canceled do not log to spa history
+		 * log as the pool may be in the process of being removed.
+		 */
+		zfs_dbgmsg("L2ARC rebuild aborted, restored %llu blocks",
+		    zfs_refcount_count(&dev->l2ad_lb_count));
 	} else if (err != 0) {
 		spa_history_log_internal(spa, "L2ARC rebuild", NULL,
 		    "aborted, restored %llu blocks",