mlxsw: spectrum: Add extack messages to VXLAN FDB rejection

Annotate the rejections in mlxsw_sp_switchdev_vxlan_work_prepare() with
textual reasons.

Because this code ends up being invoked for FDB replay as well, drop the
default message from there, so that the more accurate error message is
not overwritten.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>