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/lua/llex.c

@@ -477,7 +477,7 @@ static int llex (LexState *ls, SemInfo *seminfo) {
         else if (!lisdigit(ls->current)) return '.';
         /* else go through */
       }
-        /* FALLTHROUGH */
+        fallthrough;
       case '0': case '1': case '2': case '3': case '4':
       case '5': case '6': case '7': case '8': case '9': {
         read_numeral(ls, seminfo);