btrfs-progs: rework calculations of fi usage

btrfs-progs: rework calculations of fi usage

This patch reworks the basic calculations of 'fi usage'. It does not address
all problems but should make the code more prepared to do so.

The original code tries to estimate the free space that could lead to negative
numbers for some raid profiles:

Data, RAID1: total=147.00GiB, used=141.92GiB
System, RAID1: total=32.00MiB, used=36.00KiB
Metadata, RAID1: total=2.00GiB, used=1.17GiB
GlobalReserve, single: total=404.00MiB, used=0.00B

Overall:
    Device size:                 279.46GiB
    Device allocated:            298.06GiB
    Device unallocated:           16.00EiB
    Used:                        286.18GiB
    Free (estimated):              8.00EiB      (min: 8.00EiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              404.00MiB      (used: 0.00B)

Eg. "Device size" - "Device allocated" = negative number or a very large
positive, hence the EiB values.

There are logical and raw numbers multiplied by ratios mixed together,
so the new code makes it explicit which kind is being used. The data and
metadata ratios are calculated separately.

Output after this patch will look like:

Overall:
    Device size:                 558.92GiB
    Device allocated:            298.06GiB
    Device unallocated:          260.86GiB
    Used:                        286.18GiB
    Free (estimated):            135.51GiB      (min: 135.51GiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              404.00MiB      (used: 0.00B)

Data,RAID1: Size:147.00GiB, Used:141.92GiB
   /dev/sdc      147.00GiB
   /dev/sdd      147.00GiB

Metadata,RAID1: Size:2.00GiB, Used:1.17GiB
   /dev/sdc        2.00GiB
   /dev/sdd        2.00GiB

System,RAID1: Size:32.00MiB, Used:36.00KiB
   /dev/sdc       32.00MiB
   /dev/sdd       32.00MiB

Unallocated:
   /dev/sdc      130.43GiB
   /dev/sdd      130.43GiB

Changes:
* Device size is now the raw size, same for the following three
* Free is the logical size
* Max/min were reduced to just min

Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc        280G  144G  141G  51% /mnt/sdc

The difference between Avail and Free is there because userspace tool does a
different guesswork than kernel.

Issues not addressed by this patch:
* RAID56 profiles are not handled
* mixed profiles are not handled

Signed-off-by: David Sterba <dsterba@suse.cz>