net/mlx5: allow meta modifications in legacy mode

net/mlx5: allow meta modifications in legacy mode

The MODIFY_FIELD RTE action rejects copy to/from metadata
in case of the legacy mode extensive flow metadata support.
It is not consistent with SET_META action that has no such
restriction imposed. Registers A or B are used for META in
legacy mode. Allow meta modifications in legacy mode as well.

On other hand, SET_META rejects actions in case register C
is not available even though it is not needed in legacy mode.
Skip this check for legacy mode and allow setting META.

Fixes: edf325d421e8 ("net/mlx5: check extended metadata for meta modification")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>