remove unused variable in pbuf sys_mbox_new return error when rte ring create failed (cherry picked from commit a151525b147829163f4349b0b5f9f9011a63f525)
(cherry picked from commit 63c40c2ad66e5b32d96cecad0b123db28783ae5c)
(cherry picked from commit c312a40cbfe8b8edf4b1ab4853052952c85ab9ed)