btrfs: refactor error handling in btrfs_submit_bio

Add a bbio local variable and to prepare for calling functions that
return a blk_status_t, rename the existing int used for error handling
so that ret can be reused for the blk_status_t, and a label that can be
reused for failing the passed in bio.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>