fs: move sb_init_dio_done_wq out of direct-io.c

fs: move sb_init_dio_done_wq out of direct-io.c

sb_init_dio_done_wq is also used by the iomap code, so move it to
super.c in preparation for building direct-io.c conditionally.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230125065839.191256-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>