nfp: flower: fix pre_tun mask id allocation

pre_tun_rule flows does not follow the usual add-flow path, instead
they are used to update the pre_tun table on the firmware. This means
that if the mask-id gets allocated here the firmware will never see the
"NFP_FL_META_FLAG_MANAGE_MASK" flag for the specific mask id, which
triggers the allocation on the firmware side. This leads to the firmware
mask being corrupted and causing all sorts of strange behaviour.

Fixes: f12725d9

 ("nfp: flower: offload pre-tunnel rules")
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>