sd: notify block layer when using temporary change to cache_type

This is a fix for commit 39c60a09

  "sd: fix array cache flushing bug causing performance problems"

We must notify the block layer via q->flush_flags after a temporary change
of the cache_type to write through.  Without this, a SYNCHRONIZE CACHE
command will still be generated.  This patch factors out a helper that
can be called from sd_revalidate_disk and cache_type_store.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>