buf_flush_LRU_list_batch(): Do not skip pages that are actually clean
but in buf_pool.flush_list due to the "lazy removal" optimization of
commit 22b62eda, but try to evict them.
After acquiring buf_pool.flush_list_mutex, reread oldest_modification
to ensure that the block still remains in buf_pool.flush_list.

In addition to server hangs, this bug could also cause
InnoDB: Failing assertion: list.count > 0
in invocations of UT_LIST_REMOVE(flush_list, ...).

This fixes a regression that was caused by
commit a55b951e
and possibly made more likely to hit due to
commit aa719b50.