dlm: return -ENOENT if no comm was found

Currently if no comm can be found dlm_comm_seq() returns -EEXIST which
means entry already exists for a lookup it makes no sense to return
-EEXIST. We change it to -ENOENT. There is no user that will evaluate
the return value on a specific value so this should be fine.

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