As the part of getting extra mirror in __btrfs_map_block is
independent, this puts it into a separate function.
Reviewed-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>