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

lib/libzfsbootenv/lzbe_device.c

@@ -63,7 +63,7 @@ lzbe_set_boot_device(const char *pool, lzbe_flags_t flag, const char *device)
 			/* Drop this nvlist */
 			fnvlist_free(nv);
 		}
-		/* FALLTHROUGH */
+		fallthrough;
 	case lzbe_replace:
 		nv = fnvlist_alloc();
 		break;