mismerged log spacemap comment for metaslab_verify_weight_and_frag

When the log spacemap commit was merged in ZoL, the
metaslab_verify_unflushed_changes() debugging function
was deleted as the feature was pretty much stable by
then. Unfortunately though there was a reference to
it from a comment in metaslab_verify_weight_and_frag().

This patch deletes the reference and pastes that
comment as is.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #9097

module/zfs/metaslab.c

@@ -1639,7 +1639,15 @@ metaslab_verify_weight_and_frag(metaslab_t *msp)
 	if ((zfs_flags & ZFS_DEBUG_METASLAB_VERIFY) == 0)
 		return;
 
-	/* see comment in metaslab_verify_unflushed_changes() */
+	/*
+	 * We can end up here from vdev_remove_complete(), in which case we
+	 * cannot do these assertions because we hold spa config locks and
+	 * thus we are not allowed to read from the DMU.
+	 *
+	 * We check if the metaslab group has been removed and if that's
+	 * the case we return immediately as that would mean that we are
+	 * here from the aforementioned code path.
+	 */
 	if (msp->ms_group == NULL)
 		return;