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/lgc.c

@@ -676,7 +676,7 @@ static void freeobj (lua_State *L, GCObject *o) {
     case LUA_TUSERDATA: luaM_freemem(L, o, sizeudata(gco2u(o))); break;
     case LUA_TSHRSTR:
       G(L)->strt.nuse--;
-      /* FALLTHROUGH */
+      fallthrough;
     case LUA_TLNGSTR: {
       luaM_freemem(L, o, sizestring(gco2ts(o)));
       break;