dlm: validate messages before processing

There was some hit and miss validation of messages that has now been
cleaned up and unified.  Before processing a message, the new
validate_message() function checks that the lkb is the appropriate type,
process-copy or master-copy, and that the message is from the correct
nodeid for the the given lkb.  Other checks and assertions on the
lkb type and nodeid have been removed.  The assertions were particularly
bad since they would panic the machine instead of just ignoring the bad
message.

Although other recent patches have made processing old message unlikely,
it still may be possible for an old message to be processed and caught
by these checks.

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