ublk vs write cache and FUA

[Christoph Hellwig <hch@xxxxxx>]

Hi Ming,

ublk implements REQ_OP_FLUSH and REQ_FUA, but will never see those as
it never calls blk_queue_write_cache.  Can we drop the code?  Or should
there be a flag to enable write cache and fua support when setting up
a device?