Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

module/zfs/vdev_label.c

@@ -1305,7 +1305,7 @@ vdev_label_read_bootenv(vdev_t *rvd, nvlist_t *bootenv)
 				nvlist_free(config);
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			/* Check for FreeBSD zfs bootonce command string */
 			buf = abd_to_buf(abd);