net/mlx5: Limit scope of dump_fill_mkey function

mlx5_core_dump_fill_mkey() is going to be used in next
patch in IB and doesn't need to be visible to whole
mlx5_core. Move that command to mlx5_ib.

Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>