Btrfs: search parity device wisely

After mapping block with BTRFS_MAP_WRITE, parities have been sorted to
the end position, so this search can start from the first parity
stripe.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ copied changelog as a comment ]
Signed-off-by: David Sterba <dsterba@suse.com>