This is due to the for-loop in btr_drop_ahi_for_table() will skip
pages of status BUF_IO_WRITE, and re-check it again later. But there
are some other reasons the status can't be changed to BUF_IO_NONE quickly.
Thus it continues to iterate over LRU list over and over. This may
last for severl seconds or even minutes.

Current fix is to relax the condition to drop AHI for pages of
BUF_IO_WRITE too, and it will yield for a while after every iteration
over all LRU lists.

Reviewed-by: Jimmy Yang <[email protected]>