net/mlx5e: Wait for concurrent flow deletion during neigh/fib events

Function mlx5e_take_tmp_flow() skips flows with zero reference count. This
can cause syndrome 0x179e84 when the called from neigh or route update code
and the skipped flow is not removed from the hardware by the time
underlying encap/decap resource is deleted. Add new completion
'del_hw_done' that is completed when flow is unoffloaded. This is safe to
do because flow with reference count zero needs to be detached from
encap/decap entry before its memory is deallocated, which requires taking
the encap_tbl_lock mutex that is held by the event handlers code.

Fixes: 8914add2

 ("net/mlx5e: Handle FIB events to update tunnel endpoint device")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>