Btrfs: remove unnecessary code of stripe_index assignment in __btrfs_map_block

stripe_index's value was set again in latter line:
stripe_index = 0;

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>