[S390] qdio: make sure data structures are correctly aligned.

The slsb structure contained at the beginning of the qdio_q structure
must start on a 256 byte boundary. To make sure this is the case even
if slab debugging is turned on create an own slab cache for qdio_q
structures.
Besides that don't use the slab allocator to allocate whole pages. Use
the page allocator instead.
Also fix a few memory leaks in error handling code.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>