Currently the reason for rejection of PVID manipulation is dumped to
syslog, and a generic -EBUSY is returned to the userspace. But
switchdev_handle_port_obj_add(), through which we get to
mlxsw_sp_port_vlans_add(), handles extack just fine, and we can pass the
message this way.

This improves visibility into reasons why the request to change PVID
was rejected. Before the change:

 # bridge vlan add dev br vid 2 self pvid untagged
 RTNETLINK answers: Device or resource busy
 (plus a syslog line)

After the change:

 # bridge vlan add dev br vid 2 self pvid untagged
 Error: mlxsw_spectrum: Can't change PVID, it's used by router interface.

Note that this particular error message is going away in the following
patches. However the ability to pass error messages through extack will be
useful more broadly for communicating in particular reasons why a RIF
failed to be created.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>