nvme: Use libnvme sysfs function to get NVMe controller directory

[ikegami-t]

Also delete the SYS_NVME definition unused instead.

[ikegami-t]

Depended on the libnvme PR changes linux-nvme/libnvme#801.

[igaw]

As I wrote in PR I think we need a good reason for making sysfs configurable in nvme-cli. libnvme has unit tests which rely on it. But for nvme-cli we don't have those and for this particular function here it's not going to work because of the mmap operation.

[ikegami-t]

Okay I see. Understood as the sysfs functions purpose is differrent for the patch since I though just change nvme-cli not use the sysfs paths directly but libnvme provides them. I will do consider this more later but anyway let me close the PR. Thank you for your reviewing and comments.