lua: cast through uintptr_t when return a pointer

Don't assume size_t can carry pointer provenance and use uintptr_t
(identialy on all current platforms) instead.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes openzfs#14131

module/lua/lapi.c

@@ -444,7 +444,7 @@ LUA_API const void *lua_topointer (lua_State *L, int idx) {
     case LUA_TTABLE: return hvalue(o);
     case LUA_TLCL: return clLvalue(o);
     case LUA_TCCL: return clCvalue(o);
-    case LUA_TLCF: return cast(void *, cast(size_t, fvalue(o)));
+    case LUA_TLCF: return cast(void *, cast(uintptr_t, fvalue(o)));
     case LUA_TTHREAD: return thvalue(o);
     case LUA_TUSERDATA:
     case LUA_TLIGHTUSERDATA: