net/cxgbe: fix dangling pointer by mailbox access rework

net/cxgbe: fix dangling pointer by mailbox access rework

Rework mailbox access serialization to dynamically allocate and
free mbox entry. Also remove unnecessary temp memory and macros.

Observed with: gcc-12.0 (GCC) 12.0.1 20220118 (experimental)

In file included from ../lib/eal/linux/include/rte_os.h:14,
                 from ../lib/eal/include/rte_common.h:28,
                 from ../lib/eal/include/rte_log.h:25,
                 from ../lib/ethdev/rte_ethdev.h:164,
                 from ../lib/ethdev/ethdev_driver.h:18,
                 from ../drivers/net/cxgbe/base/t4vf_hw.c:6:
In function ‘t4_os_atomic_add_tail’,
    inlined from ‘t4vf_wr_mbox_core’ at
	../drivers/net/cxgbe/base/t4vf_hw.c:115:2:
../drivers/net/cxgbe/base/adapter.h:742:9:
      warning: storing the address of local variable ‘entry’ in
      ‘((struct mbox_list *)adapter)[96].tqh_last’ [-Wdangling-pointer=]
  742 |         TAILQ_INSERT_TAIL(head, entry, next);
      |         ^~~~~~~~~~~~~~~~~
../drivers/net/cxgbe/base/t4vf_hw.c: In function ‘t4vf_wr_mbox_core’:
../drivers/net/cxgbe/base/t4vf_hw.c:86:27: note: ‘entry’ declared here
   86 |         struct mbox_entry entry;
      |                           ^~~~~
../drivers/net/cxgbe/base/t4vf_hw.c:86:27: note: ‘adapter’ declared here

Fixes: 3bd122eef2cc ("cxgbe/base: add hardware API for Chelsio T5 series adapters")
Cc: stable@dpdk.org
Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>