s390/zcrypt: fix kmalloc 256k failure

commit b6186d7f upstream.

Tests showed that under stress conditions the kernel may
temporary fail to allocate 256k with kmalloc. However,
this fix reworks the related code in the cca_findcard2()
function to use kvmalloc instead.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>