lib-index: Fix detecting whether caller synced everything in mailbox.

lib-index: Fix detecting whether caller synced everything in mailbox.

When mailbox_index_sync_begin() was followed by _commit(), without _next()s
in the middle actually syncing the mailbox, the tail_offset was updated to
indicate that the mailbox was fully synced. Existing code didn't rely on
this, so it probably didn't break anything.

This code hasn't worked for a long time, because log_view is always read
fully to the end in _sync_begin().