btrfs-progs: fix wrong data ratio for raid56 in btrfs-file-usage

btrfs-progs: fix wrong data ratio for raid56 in btrfs-file-usage

When run btrfs-file-usage on a btrfs with data profile raid5/6,
the output message for "Free" & "Data to device ratio" seems wrong
as follows:
    ...
    Device size:		 100.00GiB
    Device allocated:		   2.04GiB
    Device unallocated:		  97.96GiB
    Used:			   1.12MiB
    Free (Estimated):		 197.89GiB <== Free > Device size
    Data to device ratio:	     198 % <== > 100%
    Global reserve:		     0.00B
    ...

It is because the function get_raid56_used() is not iterating the
chunk_info array correctly, it is just repeating adding the first
chunk_info statistics.
Just add a ptr to iterate over the array.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>