tipc: Cosmetic changes to neighbor discovery logic

Reworks the appearance of the routine that processes incoming
LINK_CONFIG messages to keep the main logic flow at a consistent level
of indentation, and to add comments outlining the various phases involved
in processing each message. This rework is being done to allow upcoming
enhancements to this routine to be integrated more cleanly.

The diff isn't really readable, so know that it was a case of the
old code being like:

	tipc_disc_recv_msg(..)
	{
		if (in_own_cluster(orig)) {
			...
			lines and lines of stuff
			...
		}
	}

which is now replaced with the more sane:

	tipc_disc_recv_msg(..)
	{
		if (!in_own_cluster(orig))
			return;
		...
		lines and lines of stuff
		...
	}

Instances of spin locking within the reindented block were replaced with
the identical tipc_node_[un]lock() abstractions.  Note that all these
changes are cosmetic in nature, and do not change the way LINK_CONFIG
messages are processed.

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