mlxsw: spectrum_router: Fix failure caused by double fib removal from HW

In mlxsw we squash tables 254 and 255 together into HW. Kernel adds/dels
/32 ip to/from both 254 and 255. On del path, that causes the same
prefix being removed twice. Fix this by introducing reference counting
for private mlxsw fib entries. That required a bit of code reshuffle.
Also put dev into fib entry key so the same prefix could be represented
once per every router interface.

Fixes: 61c503f9 ("mlxsw: spectrum_router: Implement fib4 add/del switchdev obj ops")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>