route/fib_algo: Fix nexthop reference leak on FLM_BATCH

[pouria]

Summary

handle_rtable_change_cb() references the new nexthop before calling
the algorithm. If the algorithm asks to batch the change by
returning FLM_BATCH, queue_rtable_change() references the same
nexthop again for the queued entry, while apply_rtable_changes()
only unreferences the old nexthop of each entry.

Reference new nhop once in handle_rtable_change_cb(), before
deciding whether the change is queued or passed to the algorithm,
and do not reference it again in fill_change_entry(). This also
leaves the queue reallocation as the only way for
queue_rtable_change() to fail.

Currently, dxr is the only algorithm returning FLM_BATCH.

Fixes: 6b8ef0d428c9 ("Add batched update support for the fib algo.")
MFC after: 3 days

Test Plan

kldload -n fib_dxr
ifconfig $(ifconfig epair create) inet 192.0.2.1/24 up

# Select dxr for IPv4
sysctl net.route.algo.inet.algo=dxr

# Add a route through a gateway nothing else uses, then delete it
route -n add -net 198.51.100.0/24 192.0.2.4
sleep 2
route -n delete -net 198.51.100.0/24
sleep 2

# The route is gone, but its nexthop is still there
netstat -rn4 | grep 198.51.100
netstat -onW4 | grep ' 192.0.2.4 '

[pouria]

Remove stale comment from prev patch