xen-blkfront: don't disable cache flushes when they fail

xen-blkfront: don't disable cache flushes when they fail

blkfront always had a robust negotiation protocol for detecting a write
cache.  Stop simply disabling cache flushes in the block layer as the
flags handling is moving to the atomic queue limits API that needs
user context to freeze the queue for that.  Instead handle the case
of the feature flags cleared inside of blkfront.  This removes old
debug code to check for such a mismatch which was previously impossible
to hit, including the check for passthrough requests that blkfront
never used to start with.

Signed-off-by: Christoph Hellwig <hch@lst.de>