btrfs: rename do_chunk_alloc to btrfs_chunk_alloc

Really we just need the enum, but as we break more things up it'll help
to have this external to extent-tree.c.

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