s390/qdio: Move memory alloc/pointer arithmetic for slib and sl into one place

s390/qdio: Move memory alloc/pointer arithmetic for slib and sl into one place

Instead of distributing the memory allocation and pointer arithmetic to
place slib and sl on the page that is allocated for them over multiple
functions and comments, move both into the same context directly next to
each other, so that the knowledge of how this is done is immediately
visible.

The actual layout in memory doesn't change with this, just the structure
of the code to achieve it.

Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>