nvme: only reconfigure discard if necessary

Currently nvme reconfigures discard for every disk revalidation. This
is problematic because any O_WRONLY or O_RDWR open will trigger a
partition scan through udev/systemd, and we will reconfigure discard.
This blows away any user settings, like discard_max_bytes.

Only re-configure the user settable settings if we need to.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
[removed redundant queue flag setting]
Signed-off-by: Keith Busch <keith.busch@intel.com>