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