dlm: Make dismatch error message more clear

This change will try to make this error message more clear,
since the upper applications (e.g. ocfs2) invoke dlm_new_lockspace
to create a new lockspace with passing a cluster name. Sometimes,
dlm_new_lockspace return failure while two cluster names dismatch,
the user is a little confused since this line error message is not
enough obvious.

Signed-off-by: Gang He <ghe@suse.com>
Signed-off-by: David Teigland <teigland@redhat.com>