fs: btrfs: inode: Allow next_length() to return value > BTRFS_NAME_LEN

All existing next_length() caller handles return value > BTRFS_NAME_LEN,
so there is no need to do BTRFS_NAME_LEN check in next_length().

But still, we want to exit early if we're beyond BTRFS_NAME_LEN, so this
patch makes next_length() exit as soon as we're beyond BTRFS_NAME_LEN.

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