tipc: recode getsockopt error handling for better readability

The existing code for the copy to user and error handling at the
end of getsockopt isn't easy to follow, due to the excessive use
of if/else.  By simply using return where appropriate, it can be
made smaller and easier to follow at the same time.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>