NVMe: Implement Flush

Linux implements Flush as a bit in the bio.  That means there may also be
data associated with the flush; if so the flush should be sent before the
data.  To avoid completing the bio twice, I add CMD_CTX_FLUSH to indicate
the completion routine should do nothing.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>