btrfs: Define submit_bio_hook's type directly

There is no need to use a typedef to define the type of the function and
then use that to define the respective member in extent_io_ops.  Define
struct's member directly. No functional changes.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>