fs: dlm: handle rcom in else if branch

Currently we handle in dlm_receive_buffer() everything else than a
DLM_MSG type as DLM_RCOM message. Although a different message than
DLM_MSG should be a DLM_RCOM we should explicit check on DLM_RCOM and
drop a log_error() if we see something unexpected.

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