mlxsw: spectrum_router: Use one LPM tree for all virtual routers

The number of LPM trees available for lookup is much smaller than the
number of virtual routers, which are used to implement VRFs. In
addition, an LPM tree can only be used by one protocol - either IPv4 or
IPv6.

Therefore, in order to increase the number of supported virtual routers
to the maximum we need to be able to share LPM trees across virtual
routers instead of trying to find an optimized tree for each.

Do that by allocating one LPM tree for each protocol, but make sure it
will only include prefixes that are actually used, so as to not perform
unnecessary lookups.

Since changing the structure of a bound tree isn't recommended, whenever
a new tree it required, it's first created and then bound to each
virtual router, replacing the old one.

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>