ndp: Accept multiple queued ND for non-GRAND NAs

[pouria]

Summary

Multiple delayed NAs on the same ifa can occur simultaneously.
Therefore:

• Differentiate between GRAND and solicited replies.
• Cancel previous pending GRAND NA for same ifa.
• Reuse ndq memory for GRAND.
• Free non-GRAND replies immediately.
• Don't limit non-GRAND NAs.

Test Plan

Increase grand count before running tests:

sysctl net.inet6.ip6.grand_count=10

then, simply run kyua network tests:

kyua -k /usr/tests/Kyuafile sys/net
kyua -k /usr/tests/Kyuafile sys/netinet6
kyua -k /usr/tests/Kyuafile sys/netlink
kyua -k /usr/tests/Kyuafile sys/netgraph

or you can add multiple v6 addresses followed by changing
the mac address of interface.
Do it in jail and destroy it for stress.

[pouria]

Don't count for grand for delayed solicited NAs.

[pouria]

Reuse ndq and cancel out previous pending NA for same ifa.
@glebius done.

[pouria]

Fix a minor issue on PATH_MASK.

[glebius]

sys/netinet6/nd6_nbr.c

1755–1770

This logic can be achieved without new boolean, just using blocks. Also we need not to leak a lock in case of malloc(9) failure.

1863–1864

I would say that previous version was easier to read. A single version of nd6_queue_rel() that requires lock on entry and returns unlocked. Then in this cycle use return; instead of break;. Basically nd6_queue_stop() doesn't need any changes. And second version of nd6_queue_rel_locked() isn't needed.

[glebius]

I'd suggest to use more specific commit line, e.g. ndp: handle a case when two timers coalesce for the same ifa. Or something else that you can come up with. Just more specific than "make things better" :)

[pouria]

Address @glebius comments and update according to our conversation.

ndp: Accept multiple queued ND for non-GRAND NAs

Multiple delayed NAs on the same ifa can occur simultaneously.
Therefore:
* Differentiate between GRAND and solicited replies.
* Cancel previous pending GRAND NA for same ifa.
* Reuse ndq memory for GRAND.
* Free non-GRAND replies immediately.
* Don't limit non-GRAND NAs.

See also D55927 for one the tests.