[PATCH 0/2][v2] Rework the worst case calculations for space reservation

[Josef Bacik <josef@toxicpanda.com>]

v1->v2:
- dropped "btrfs: global reserve fallback should use metadata_size", turns out
  I was testing without my evict changes in place so we don't even need this in
  the first place, but it is also wrong because we need to reserve space for the
  orphan item which is an insert.
- Added the reviewed-by's.

-- Original email --
We have two worst case calculations for space reservation, one that takes into
account splitting at every level when cow'ing down the btree, and another that
doesn't account for splitting at all.  The first is used everywhere, and the
second is used mostly for truncate.

However we also do not split when we're only changing an item, so for example
updating the inode item.  So the name for this helper is wrong, because it can
be used for in-place updates as well as for truncates.  Rename the helpers and
then use the smaller worst-case reservation for inode updates in a few places.

As a rule we still want to use the insert calculation when we can't be sure what
kind of operation is going to end up happening.  But for things like delayed
inode updates and file writes where we know there is going to be an existing
inode item we can use the smaller reservation.  Thanks,

Josef