net/rxrpc: Use an IS_ERR test rather than a NULL test

In case of error, the function rxrpc_get_transport returns an ERR
pointer, but never returns a NULL pointer. So after a call to this
function, a NULL test should be replaced by an IS_ERR test.

A simplified version of the semantic patch that makes this change is
as follows: 
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@correct_null_test@
expression x,E;
statement S1, S2;
@@
x =  rxrpc_get_transport(...)
<... when != x = E
if (
(
- x@p2 != NULL
+ ! IS_ERR ( x )
|
- x@p2 == NULL
+ IS_ERR( x )
)
 )
S1
else S2
...>
? x = E;
// </smpl>

Signed-off-by: Julien Brunel <brunel@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>