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/ltable.c

@@ -491,7 +491,7 @@ const TValue *luaH_get (Table *t, const TValue *key) {
         return luaH_getint(t, k);  /* use specialized version */
       /* else go through */
     }
-      fallthrough;
+      zfs_fallthrough;
     default: {
       Node *n = mainposition(t, key);
       do {  /* check whether `key' is somewhere in the chain */