tipc: Discard incoming broadcast messages that are unexpected

tipc: Discard incoming broadcast messages that are unexpected

Modifies TIPC's incoming broadcast packet handler to discard messages
that cannot legally be sent over the broadcast link, including:

- broadcast protocol messages that do no contain state information
- payload messages that are not named multicast messages
- any other form of message except for bundled messages, fragmented
  messages, and name distribution messages.

These checks are needed to prevent TIPC from handing an unexpected
message to a routine that isn't prepared to handle it, which could
lead to incorrect processing (up to and including invalid memory
references caused by attempts to access message fields that aren't
present in the message).

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>