btrfs: statfs: report zero available if metadata are exhausted

There is one ENOSPC case that's very confusing. There's Available
greater than zero but no file operation succeds (besides removing
files). This happens when the metadata are exhausted and there's no
possibility to allocate another chunk.

In this scenario it's normal that there's still some space in the data
chunk and the calculation in df reflects that in the Avail value.

To at least give some clue about the ENOSPC situation, let statfs report
zero value in Avail, even if there's still data space available.

Current:
  /dev/sdb1             4.0G  3.3G  719M  83% /mnt/test

New:
  /dev/sdb1             4.0G  3.3G     0 100% /mnt/test

We calculate the remaining metadata space minus global reserve. If this
is (supposedly) smaller than zero, there's no space. But this does not
hold in practice, the exhausted state happens where's still some
positive delta. So we apply some guesswork and compare the delta to a 4M
threshold. (Practically observed delta was 2M.)

We probably cannot calculate the exact threshold value because this
depends on the internal reservations requested by various operations, so
some operations that consume a few metadata will succeed even if the
Avail is zero. But this is better than the other way around.

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