btrfs: Remove redundant bio_get/bio_set pair from submit_one_bio

The bio is never referenced after it has been submitted so there is no
point in getting an extra reference.

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