Tidy up callout for select(2) and read timeout.
Tidy up callout for select(2) and read timeout.

- Add a missing callout_drain(9) before the descriptor deallocation.[1]
- Prefer callout_init_mtx(9) over callout_init(9) and let the callout
subsystem handle the mutex for callout function.

PR:		kern/144453
Submitted by:	Alexander Sack (asack at niksun dot com)[1]
MFC after:	1 week

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