NVMe: Configure support for block flush

This configures an nvme request_queue as flush capable if the device
has a volatile write cache present.

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

diff --cc drivers/block/nvme-core.c
Simple merge

diff --cc include/linux/nvme.h
Simple merge

diff --cc include/uapi/linux/nvme.h
Simple merge