[PATCH V2] btrfs-progs: add RAID5/6 support to btrfs fi us

[Goffredo Baroncelli <kreijack@libero.it>]

Hi all,

this patch adds support for the raid5/6 profiles in the command
'btrfs filesystem usage'.

Until now the problem was that the value r_{data,metadata}_used is not
easy to get for a RAID5/6, because it depends by the number of disks.
And in a filesystem it is possible to have several raid5/6 chunks with a
different number of disks.

In order to bypass this issue, I compute these values from the 
r_{data,metadata,system}_chunks values and the ratio
l_*_used / l_*_chunks.

So now if you run btrfs fi us in a raid6 filesystem you get:

$ sudo btrfs fi us / 
Overall:
    Device size:		  40.00GiB
    Device allocated:		   8.28GiB
    Device unallocated:		  31.72GiB
    Device missing:		     0.00B
    Used:			   5.00GiB
    Free (estimated):		  17.36GiB	(min: 17.36GiB)
    Data ratio:			      2.00
    Metadata ratio:		      0.00
    Global reserve:		   3.25MiB	(used: 0.00B)

Data,RAID6: Size:4.00GiB, Used:2.50GiB (62.53%)
[...]

Instead before:

$ sudo btrfs fi us /
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
WARNING: RAID56 detected, not implemented
Overall:
    Device size:		  40.00GiB
    Device allocated:		     0.00B
    Device unallocated:		  40.00GiB
    Device missing:		     0.00B
    Used:			     0.00B
    Free (estimated):		     0.00B	(min: 8.00EiB)
    Data ratio:			      0.00
    Metadata ratio:		      0.00
    Global reserve:		   3.25MiB	(used: 0.00B)

Data,RAID6: Size:4.00GiB, Used:2.50GiB (62.53%)
[...]


I want to point out that this patch should be compatible with my
previous patches set (the ones related to the new ioctl 
BTRFS_IOC_GET_CHUNK_INFO). If both are merged we will have a 'btrfs fi us'
commands with full support a raid5/6 filesystem without needing root
capability.

I rewrote the patch after some David's comments about the difficult to
review it because I changed too much code. So this time I tried to be less
intrusive. I leaved the old logic and I computed only the missing
values.

Comments are welcome.
BR
G.Baroncelli

V2:
The patch is completely rewritten to be less intrusive

V1:
First issue

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