nvme: Fix namespace parsing

[goll]

This PR fixes the namespace parsing bug introduced by #765 as the regex on L30 in class_nvme.go explicitly looks for nvme0c0n0.

Standard nvme devices will not have the c0 identifier, from what I gather that is only applicable in some multipath setups, it will just be nvme0n1 for example.

Noticed this following node_exporter upgrade that promised nvme metrics which were missing on all updated systems.

>> running all tests
go test -race  ./...
ok  	github.com/prometheus/procfs	1.132s
ok  	github.com/prometheus/procfs/bcache	1.017s
ok  	github.com/prometheus/procfs/bcachefs	1.013s
ok  	github.com/prometheus/procfs/blockdevice	1.020s
ok  	github.com/prometheus/procfs/btrfs	1.016s
?   	github.com/prometheus/procfs/ext4	[no test files]
ok  	github.com/prometheus/procfs/internal/fs	1.016s
ok  	github.com/prometheus/procfs/internal/util	(cached)
ok  	github.com/prometheus/procfs/iscsi	1.023s
ok  	github.com/prometheus/procfs/nfs	(cached)
ok  	github.com/prometheus/procfs/selinuxfs	1.015s
ok  	github.com/prometheus/procfs/sysfs	1.054s
ok  	github.com/prometheus/procfs/xfs	1.027s

[goll]

@discordianfish @SuperQ since you gentlemen handled #765

[SuperQ]

@ShashwatHiregoudar can you comment on this naming change?

[SuperQ]

LGTM, I'm not sure where the cN naming comes from. I can't find any kernel documentation evidence that it's supposed to exist this way.