Change the bounce buffer logic of cpcmd. diag8 needs _real_ memory below
2GB. Therefore vmalloced data does not work. As the data might cross a
page boundary, we cannot use virt_to_page either. The solution is to use
virt_to_page only in the check for a bounce buffer.

There was a redundant check for response==NULL. response < 2GB contains
this check as well.

I also removed the rlen==0 check, since rlen=0 and response!=NULL would
be a caller bug and response==NULL is already checked.
Signed-off-by: NChristian Borntraeger <cborntra@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>