btrfs-progs: cleanup filesystem/device usage code

btrfs-progs: cleanup filesystem/device usage code

The main point of this is to load the device and chunk infos at one
place and pass down to the printers. The EPERM is handled separately, in
case kernel does not give us all the information about chunks or
devices, but we want to warn and print at least something.

For non-root users, 'filesystem usage' prints only the overall stats and
warns about RAID5/6.

The sole cleanup changes affect mostly the modified code and the related
functions, should be reasonably small.

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