BUG#33290335 INPLACE DDL leaves pages with dangling flush observer

BUG#33238133 Assertion failure: buf0lru.cc:449:bpage->in_flush_list

Problem:
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There are many loop holes in buf_flush_dirty_pages() flow which could
result in required pages not flushed.

Issue - 1 : Re-start out parameter value is not handled after yield
Issue - 2 : While removing page, the prev page could get flushed
Issue - 3 : After yield there is a window after page is unpinned

Solution:
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The code is simplified and loop holes are fixed.

Reviewed-by: Sunny Bains <[email protected]>
Reviewed-by: Marcin Babij <[email protected]>

RB: 26962

Author: Debarun Banerjee