nvme: Change namespace device name

Changes the device name for NVMe and NVMe-oF namespaces from using "ns"
to "n" to be more compatible with other operating systems. For example,
a device which was previously /dev/nvme0ns1 is now /dev/nvme0n1.

Preserves the existing functionality by creating alias from nvmeXnY to
nvmeXnsY.

Reviewed by: imp
MFC after: 1 month
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D45414