fs: btrfs: Rename struct btrfs_path to struct __btrfs_path

To avoid name conflicting between the extent buffer based btrfs_path
from btrfs-progs, rename struct btrfs_path to struct __btrfs_path.

Also rename btrfs_free_path() to __btrfs_free_path() to avoid conflicts.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>