btrfs-progs: enable negative numbers for unallocated device space

btrfs-progs: enable negative numbers for unallocated device space

The unallocated space is not calculated correctly when a device
deltion/replace is in progress. This appears as huge number, EiB-sized.
It's really a negative number, because we don't have entire information
available and mixing numbers that do and don't take the deleted device
size into account.

We have to query search ioctl to retrieve the missing information, also
this requires root access for the ioctl, so we still need a workaround
for non-root case. Here the negative numbers seem to be a bit more
practical than EiB numbers.

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