mlxsw: spectrum_router: Free LPM tree upon failure

When a new LPM tree is created, we try to replace the trees in the
existing virtual routers with it. If we fail, the tree needs to be
freed.

Currently, this does not happen in the unlikely case where we fail to
bind the tree to the first virtual router, since its reference count
never transitions from 1 to 0.

Fix that by taking a reference before binding the tree.

Fixes: fc922bb0

 ("mlxsw: spectrum_router: Use one LPM tree for all virtual routers")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>