s390/zcrypt: fix using the correct variable for sizeof()

While the original code is valid, it is not the obvious choice for the
sizeof() call and in preparation to limit the scope of the list iterator
variable the sizeof should be changed to the size of the variable
being allocated.

Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>