eventfd: make eventfd_signal{_mask}() void

No caller care about the return value.

Link: https://lore.kernel.org/r/20231122-vfs-eventfd-signal-v2-4-bd549b14ce0c@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>