iomap: allow the file system to provide a bio_set for direct I/O

Allow the file system to provide a specific bio_set for allocating
direct I/O bios.  This will allow file systems that use the
->submit_io hook to stash away additional information for file system
use.

To make use of this additional space for information in the completion
path, the file system needs to override the ->bi_end_io callback and
then call back into iomap, so export iomap_dio_bio_end_io for that.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>