sleepqueue: Address a lock order reversal

After commit 74cf7cae4d22 ("softclock: Use dedicated ithreads for
running callouts."), there is a lock order reversal between the per-CPU
callout lock and the scheduler lock. softclock_thread() locks callout
lock then the scheduler lock, when preparing to switch off-CPU, and
sleepq_remove_thread() stops the timed sleep callout while potentially
holding a scheduler lock. In the latter case, it's the thread itself
that's locked, and if the thread is sleeping then its lock will be a
sleepqueue lock, but if it's still in the process of going to sleep
it'll be a scheduler lock.

We could perhaps change softclock_thread() to try to acquire locks in
the opposite order, but that'd require dropping and re-acquiring the
callout lock, which seems expensive for an operation that will happen
quite frequently. We can instead perhaps avoid stopping the
td_slpcallout callout if the thread is still going to sleep, which is
what this patch does. This will result in a spurious call to
sleepq_timeout(), but some counters suggest that this is very rare.

PR: 261198
Fixes: 74cf7cae4d22 ("softclock: Use dedicated ithreads for running callouts.")
Reported and tested by: thj
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34204