rules: block: add support for pmem devices (#3683)

rules: block: add support for pmem devices (#3683)

Persistent memory devices can be exposed as block devices as /dev/pmemN
and /dev/pmemNs.  pmemN is the raw device and is byte-addressable from
within the kernel and when mmapped by applications from a DAX-mounted
file system.  pmemNs has the block translation table (BTT) layered on top,
offering atomic sector/block access.  Both pmemN and pmemNs are expected
to contain file systems.

blkid(8) and lsblk(8) seem to correctly report on pmemN and pmemNs.
systemd v219 will populate /dev/disk/by-uuid/ when, for example, mkfs is
used on pmem, but systemd v228 does not.

Add pmem to the whitelist.