net/i40e: fix virtual channel conflict

net/i40e: fix virtual channel conflict

i40evf_execute_vf_cmd() uses _atomic_set_cmd() to execute virtual
channel commands safely in multi-process mode and multi-thread mode.
However, it returns error when one process or thread is pending. Add
rte_spinlock_trylock() to handle this issue in concurrent scenarios.

Fixes: 4861cde46116 ("i40e: new poll mode driver")
Cc: stable@dpdk.org
Signed-off-by: Yuying Zhang <yuying.zhang@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>