From Dave's testing described below, it's possible to drive a file
system to have bogus values of discardable_extents and _bytes.  As
btrfs_discard_calc_delay() is the only user of discardable_extents, we
can correct here for any negative discardable_extents/discardable_bytes.

The problem is not reliably reproducible. The workload that created it
was based on linux git tree, switching between release tags, then
everytihng deleted followed by a full rebalance. At this state the
values of discardable_bytes was 16K and discardable_extents was -1,
expected values 0 and 0.

Repeating the workload again did not correct the bogus values so the
offset seems to be stable once it happens.
Reported-by: David Sterba <dsterba@suse.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>