We can simplify the code gracefully with new guard() macro and co for
automatic cleanup of locks.

For making changes easier, some functions widen the application of
register_mutex, but those shouldn't influence on any actual
performance.

Also, one code block was factored out as a function so that guard()
can be applied cleanly without much indentation.

There are still a few remaining explicit spin_lock/unlock calls, and
those are for the places where we do temporary unlock/relock, which
doesn't fit well with the guard(), so far.

Only the code refactoring, and no functional changes.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240227085306.9764-4-tiwai@suse.de