dlm: fix invalid cluster name warning

The warning added in commit 3b0e761b
  "dlm: print log message when cluster name is not set"

did not account for the fact that lockspaces created
from userland do not supply a cluster name, so bogus
warnings are printed every time a userland lockspace
is created.

Signed-off-by: David Teigland <teigland@redhat.com>