[S390] drivers/s390: Use an IS_ERR test rather than a NULL test

In case of error, functions dasd_kmalloc_request and idal_buffer_alloc
return an ERR pointer, but never return the NULL pointer. So after a
call to one of these functions, 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 =
(
  dasd_kmalloc_request(...)
|
  idal_buffer_alloc(...)
)
<... 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: Martin Schwidefsky <schwidefsky@de.ibm.com>