Block_device_wait does not return an error code

Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Elling <Richard.Elling@RichardElling.com>
Closes #8839

tests/zfs-tests/include/blkdev.shlib

@@ -56,6 +56,9 @@ function scan_scsi_hosts
 #
 # Wait for newly created block devices to have their minors created.
 #
+# Note: there is no meaningful return code if udevadm fails. Consumers
+# should not expect a return code (do not call as argument to log_must)
+#
 function block_device_wait
 {
 	if is_linux; then