net/mlx5: check metadata registers availability

net/mlx5: check metadata registers availability

The metadata registers reg_c provide support for TAG and
SET_TAG features. Although there are 8 registers are available
on the current mlx5 devices, some of them can be reserved.
The availability should be queried by iterative trial-and-error
implemented by mlx5_flow_discover_mreg_c() routine.

If reg_c is available, it can be regarded inclusively that
the extensive metadata support is possible. E.g. metadata
register copy action, supporting 16 modify header actions
(instead of 8 by default) preserving register across
different domains (FDB and NIC) and so on.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>