librbd/cache/pwl: fix bug of flush request blocked by deferd IO

librbd/cache/pwl: fix bug of flush request blocked by deferd IO

Flush requests do not need to be queued behind the defer_io queue,
should be issued immediately. Otherwise, there will be a deadlock
scenario in which dirty data is waiting for flush req, flush req is
waiting for defer_io empty, and defer_io is waiting for dirty data
persistence to release space. So this sometimes occur when the cache
is small but the IO is large or the queue depth is large.

Signed-off-by: Yin Congmin <congmin.yin@intel.com>