net/mlx5: disable dump of Verbs flows

net/mlx5: disable dump of Verbs flows

There was a segment fault when dump flows with device argument of
dv_flow_en=0. In such case, Verbs flow engine was enabled and fdb
resources were not initialized. It's suggested to use mlx_fs_dump
for Verbs flow dump.

This patch adds verbs engine check, prints warning message and return
gracefully.

Fixes: f6d7202402c9 ("net/mlx5: support flow dump API")
Cc: stable@dpdk.org
Reported-by: Jørgen Østergaard Sloth <jorgen.sloth@xci.dk>
Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>