The truncate loop in btrfs_evict_inode() does two things at once:

- It refills the temporary block reserve, potentially stealing from the
  global reserve or committing
- It calls btrfs_truncate_inode_items()

The tangle of continues hides the fact that these two steps are actually
separate. Split the first step out into a separate function both for
clarity and so that we can reuse it in a later patch.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>