tipc: refactor accept() code for improved readability

In TIPC's accept() routine, there is a large block of code relating
to initialization of a new socket, all within an if condition checking
if the allocation succeeded.

Here, we simply flip the check of the if, so that the main execution
path stays at the same indentation level, which improves readability.
If the allocation fails, we jump to an already existing exit label.

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