KVM: s390: Remove non-atomic dat_crstep_xchg()

KVM: s390: Remove non-atomic dat_crstep_xchg()

In practice dat_crstep_xchg() is racy and hard to use correctly. Simply
remove it and replace its uses with dat_crstep_xchg_atomic().

This solves some actual races that lead to system hangs / crashes.

Opportunistically fix an alignment issue in _gmap_crstep_xchg_atomic().

Fixes: 589071eaaa8f ("KVM: s390: KVM page table management functions: clear and replace")
Fixes: 94fd9b16cc67 ("KVM: s390: KVM page table management functions: lifecycle management")
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>