Btrfs: do not increment on bio_index one by one

'bio_index' is just a index, it's really not necessary to do increment
one by one.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>