s390/nmi: get rid of private slab cache

Get rid of private "nmi_save_areas" slab cache. The only reason this was
introduced years ago was that with some slab debugging options allocations
would only guarantee a minimum alignment of ARCH_KMALLOC_MINALIGN, which
was eight bytes back then. This is not sufficient for the extended machine
check save area.

However since commit 59bb4798 ("mm, sl[aou]b: guarantee natural
alignment for kmalloc(power-of-two)") kmalloc guarantees a power-of-two
alignment even with debugging options enabled.

Therefore the private slab cache can be removed.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>