mlxsw: spectrum_buffers: Manage internal buffer in the hdroom code

Traffic mirroring modes that are in-chip implemented on egress need an
internal buffer to work. As the only client, the SPAN module was managing
the buffer so far. However logically it belongs to the buffers module. E.g.
buffer size validation needs to take the size of the internal buffer into
account.

Therefore move the related code from SPAN to spectrum_buffers. Move over
the callbacks that determine the minimum buffer size as a function of
maximum speed and MTU. Add a field describing the internal buffer to struct
mlxsw_sp_hdroom. Extend mlxsw_sp_hdroom_bufs_reset_sizes() to take care of
sizing the internal buffer as well. Change the SPAN module to invoke that
function and mlxsw_sp_hdroom_configure() like all the other hdroom clients.
Drop the now-unnecessary mlxsw_sp_span_port_buffer_disable().

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>