UBIFS: double check that buds are replied in order

Commit 52c6e6f9

 provides misleading infomation
in the commit messages - buds are replied in order. And the real reason why
that fix helped is probably because it made sure we seek head even in read-only
mode (so deferred recovery will have seeked heads).

This patch adds an assertion which will fire if we reply buds out of order.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>