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

cmd/mount_zfs/mount_zfs.c

@@ -369,7 +369,7 @@ main(int argc, char **argv)
 				return (MOUNT_SYSERR);
 			}
 #endif
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) fprintf(stderr, gettext("filesystem "
 			    "'%s' can not be mounted: %s\n"), dataset,