kqueue: slightly clarify the flow in knlist_cleardel()

[kevans]

Summary

This is purely a cosmetic change to make it a little easier on the eyes,
rather than jumping back to the else branch up top. Re-flow it to use
another loop on the outside and just inline the re-lock before we repeat
after awaking from fluxwait.

The !killkn path should maybe issue a wakeup if there's a thread in
KQ_SLEEP so that userland can observe the EOF, but this isn't a
practical problem today: pretty much every case of knlist_clear is tied
to a file descriptor and called in the close(2) path. As a consequence,
potentially affected knotes are almost always destroyed before we even
get to knlist_clear().

[kib]

sys/kern/kern_event.c

2794–2818

Could this be just for (;;) ?

[kevans]

sys/kern/kern_event.c

2794–2818

I was mainly trying to avoid bothering with SLIST_FOREACH_SAFE on the last iteration if it's empty after we re-enter from fluxwait, but I don't feel that strongly about it- this isn't exactly a hot path.

[kevans]

Use an infinite loop instead, it's a little cleaner feeling.