fs: btrfs: Use btrfs_lookup_path() to implement btrfs_exists() and btrfs_size()

After this the only remaining function that still utilizes
__btrfs_lookup_path() is btrfs_read().

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