default flush to use block device nsid

default flush to use block device nsid

Kernel 5.13 added checks to ensure the ioctl path can't be abused to
access a different namespace than the one user space has permission to
open. This unfortunately breaks the all-nsid flush usage, so user
tooling needs to default to the device's namespace id.

Link: https://lore.kernel.org/linux-nvme/20210518144249.GE2709569@dhcp-10-100-145-180.wdc.com/T/#t
Link: https://github.com/linux-nvme/nvme-cli/issues/1066
Signed-off-by: Keith Busch <kbusch@kernel.org>