signal: Cleanup flush_sigqueue_mask()

Mop up the stale return value comment and add a lockdep check instead of
commenting on the locking requirement.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20241001083835.374933959@linutronix.de