mlxsw: spectrum_router: Simplify LPM tree allocation

When looking for a new LPM tree we should always consider all the unused
trees. It doesn't matter if the new tree is required due to changes in
currently used prefixes inside an existing routing table or because a
route was inserted into an empty table.

Both cases are functionally identical and therefore should be treated
the same.

When looking for a new LPM tree, consider all unused trees and don't
reserve trees for specific cases.

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>