fs: dlm: check on minimum msglen size

This patch adds an additional check for minimum dlm header size which is
an invalid dlm message and signals a broken stream. A msglen field cannot
be less than the dlm header size because the field is inclusive header
lengths.

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