fs: btrfs: Rename path resolve related functions to avoid name conflicts

Since the old code is using __btrfs_path/__btrfs_root which is different
from the regular extent buffer based one, we add "__" prefix for the old
implementation to avoid name conflicts for the incoming crossport.

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