nfs/blocklayout: refactor open-by-wwn

The current code works with the standard udev/systemd names, but we'll have
to add another method in the next patch.  Refactor it into a separate helper
to make room for the new variant.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>