net/mlx5: fix dynamic mbuf offset lookup check

net/mlx5: fix dynamic mbuf offset lookup check

The functions rte_mbuf_dynfield_lookup() and rte_mbuf_dynflag_lookup()
can return an offset starting with 0 or a negative error code.

In reality the first offsets are probably reserved forever,
but for the sake of strict API compliance,
the checks which considered 0 as an error are fixed.

Fixes: efa79e68c8cd ("net/mlx5: support fine grain dynamic flag")
Fixes: 3172c471b86f ("net/mlx5: prepare Tx queue structures to support timestamp")
Fixes: 0febfcce3693 ("net/mlx5: prepare Tx to support scheduling")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>