tipc: Fix issues with fragmentation of an existing message buffer

Modifies the routine that fragments an existing message buffer to
use similar logic to that used when generating fragments from an iovec.
The routine now creates a complete chain of fragments and adds them to
the link transmit queue as a unit, so that the link sends all fragments
or none; this prevents the incomplete transmission of a fragmented
message that might otherwise result because of link congestion or
memory exhaustion. This change also ensures that the counter recording
the number of fragmented messages sent by the link is now incremented
only if the message is actually sent.

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