btrfs: use the file extent tree infrastructure

We want to use this everywhere we modify the file extent items
permanently.  These include:

  1) Inserting new file extents for writes and prealloc extents.
  2) Truncating inode items.
  3) btrfs_cont_expand().
  4) Insert inline extents.
  5) Insert new extents from log replay.
  6) Insert a new extent for clone, as it could be past i_size.
  7) Hole punching

For hole punching in particular it might seem it's not necessary because
anybody extending would use btrfs_cont_expand, however there is a corner
that still can give us trouble.  Start with an empty file and

fallocate KEEP_SIZE 1M-2M

We now have a 0 length file, and a hole file extent from 0-1M, and a
prealloc extent from 1M-2M.  Now

punch 1M-1.5M

Because this is past i_size we have

[HOLE EXTENT][ NOTHING ][PREALLOC]
[0        1M][1M   1.5M][1.5M  2M]

with an i_size of 0.  Now if we pwrite 0-1.5M we'll increas our i_size
to 1.5M, but our disk_i_size is still 0 until the ordered extent
completes.

However if we now immediately truncate 2M on the file we'll just call
btrfs_cont_expand(inode, 1.5M, 2M), since our old i_size is 1.5M.  If we
commit the transaction here and crash we'll expose the gap.

To fix this we need to clear the file extent mapping for the range that
we punched but didn't insert a corresponding file extent for.  This will
mean the truncate will only get an disk_i_size set to 1M if we crash
before the finish ordered io happens.

I've written an xfstest to reproduce the problem and validate this fix.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>