btrfs: temporarily export alloc_extent_state helpers

We're going to move this code in stages, but while we're doing that we
need to export these helpers so we can more easily move the code into
the new file.

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