net/mlx5: fix assert in dynamic metadata handling

net/mlx5: fix assert in dynamic metadata handling

The assert in dynamic flow metadata handling is wrong after the
fix for the performance degradation. The assert meant to check
the metadata mask but was updated with the metadata offset instead.
Fix this assert and restore proper metadata mask checking.

Fixes: 6c55b622a956 ("net/mlx5: set dynamic flow metadata in Rx queues")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>