Rename fallthrough to zfs_fallthrough

Unfortunately macOS has obj-C keyword "fallthrough" in the OS headers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #13097

module/lua/lgc.c

@@ -675,7 +675,7 @@ 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;
+      zfs_fallthrough;
     case LUA_TLNGSTR: {
       luaM_freemem(L, o, sizestring(gco2ts(o)));
       break;