net/iavf: fix virtual channel return

net/iavf: fix virtual channel return

In iavf_handle_virtchnl_msg(), it is not appropriate for _clear_cmd()
to be used as a notification to foreground thread. So introduce
_notify_cmd() to fix this error. In addition, since _notify_cmd()
contains rte_wmb(), rte_compiler_barrier() is not necessary.

Sending msg from VF to PF is mainly by calling iavf_execute_vf_cmd(),
the whole virtchnl msg process is like,

iavf_execute_vf_cmd() will call iavf_aq_send_msg_to_pf() to send
msg and then polling the cmd done flag as "if (vf->pend_cmd ==
VIRTCHNL_OP_UNKNOWN)"

When reply msg is returned by pf, iavf_handle_virtchnl_msg() in
isr will read msg return value by "vf->cmd_retval = msg_ret" and
immediately set the cmd done flag by calling _clear_cmd() to
notify the iavf_execute_vf_cmd().

iavf_execute_vf_cmd() find the cmd done flag is set and then
check whether command return value vf->cmd_retval is success or
not.

However _clear_cmd() also resets the vf->cmd_retval to success,
overwriting the actual return value which is used for diagnosis.
So iavf_execute_vf_cmd() will always find vf->cmd_retval is
success and then return success.

Fixes: 22b123a36d07 ("net/avf: initialize PMD")
Cc: stable@dpdk.org
Signed-off-by: Yahui Cao <yahui.cao@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>