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/lstrlib.c

@@ -501,7 +501,7 @@ static const char *match (MatchState *ms, const char *s, const char *p) {
             }
             case '+':  /* 1 or more repetitions */
               s++;  /* 1 match already done */
-              /* FALLTHROUGH */
+              fallthrough;
             case '*':  /* 0 or more repetitions */
               s = max_expand(ms, s, p, ep);
               break;