Add 'zfs wait' command

Add a mechanism to wait for delete queue to drain.

When doing redacted send/recv, many workflows involve deleting files 
that contain sensitive data. Because of the way zfs handles file 
deletions, snapshots taken quickly after a rm operation can sometimes 
still contain the file in question, especially if the file is very 
large. This can result in issues for redacted send/recv users who 
expect the deleted files to be redacted in the send streams, and not 
appear in their clones.

This change duplicates much of the zpool wait related logic into a 
zfs wait command, which can be used to wait until the internal
deleteq has been drained.  Additional wait activities may be added 
in the future. 

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #9707

lib/libzfs/libzfs_dataset.c

@@ -5599,3 +5599,31 @@ zvol_volsize_to_reservation(zpool_handle_t *zph, uint64_t volsize,
 	volsize += numdb;
 	return (volsize);
 }
+
+/*
+ * Wait for the given activity and return the status of the wait (whether or not
+ * any waiting was done) in the 'waited' parameter. Non-existent fses are
+ * reported via the 'missing' parameter, rather than by printing an error
+ * message. This is convenient when this function is called in a loop over a
+ * long period of time (as it is, for example, by zfs's wait cmd). In that
+ * scenario, a fs being exported or destroyed should be considered a normal
+ * event, so we don't want to print an error when we find that the fs doesn't
+ * exist.
+ */
+int
+zfs_wait_status(zfs_handle_t *zhp, zfs_wait_activity_t activity,
+    boolean_t *missing, boolean_t *waited)
+{
+	int error = lzc_wait_fs(zhp->zfs_name, activity, waited);
+	*missing = (error == ENOENT);
+	if (*missing)
+		return (0);
+
+	if (error != 0) {
+		(void) zfs_standard_error_fmt(zhp->zfs_hdl, error,
+		    dgettext(TEXT_DOMAIN, "error waiting in fs '%s'"),
+		    zhp->zfs_name);
+	}
+
+	return (error);
+}