nvme: Eliminate the offset of one during block dev creation

At present there is an offset of one added during the creation of
block device. This can be very confusing as we wanted to encode the
namespace id in the block device name but namespae id cannot be zero.

This changes to use the namespace id directly in the block device
name, eliminating the offset of one effectively.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>