mlxsw: spectrum: Split headroom autoresize out of buffer configuration

Split mlxsw_sp_port_headroom_set() to three functions.
mlxsw_sp_hdroom_bufs_reset_sizes() changes the sizes of the individual PG
buffers, and mlxsw_sp_hdroom_configure_buffers() will actually apply the
configuration. A third function, mlxsw_sp_hdroom_bufs_fit(), verifies that
the requested buffer configuration matches total headroom size
requirements.

Add wrappers, mlxsw_sp_hdroom_configure() and __..., that will eventually
perform full headroom configuration, but for now, only have them verify the
configured headroom size, and invoke mlxsw_sp_hdroom_configure_buffers().
Have them take the `force` argument to prepare for a later patch, even
though it is currently unused.

Note that the loop in mlxsw_sp_hdroom_configure_buffers() only goes through
DCBX_MAX_BUFFERS. Since there is no logic to configure the control buffer,
it needs to keep the values queried from the FW. Eventually this function
should configure all the PGs.

Note that conversion of __mlxsw_sp_dcbnl_ieee_setets() is not trivial. That
function performs the headroom configuration in three steps: first it
resizes the buffers and adds any new ones. Then it redirects priorities to
the new buffers. And finally it sets the size of the now-unused buffers to
zero. This way no packet drops are introduced.

So after invoking mlxsw_sp_hdroom_bufs_reset_sizes(), tweak the
configuration to keep the old sizes of PG buffers for those buffers whose
size was set to zero.

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>