target-s390x: fix MMU index computation

The cpu_mmu_index function wrongly looks at PSW P bit to determine the
MMU index, while this bit actually only control the use of priviledge
instructions. The addressing mode is detected by looking at the PSW ASC
bits instead.

This used to work more or less correctly up to kernel 3.6 as the kernel
was running in primary space and userland in secondary space. Since
kernel 3.7 the default is to run the kernel in home space and userland
in primary space. While the current QEMU code seems to work it open some
security issues, like accessing the lowcore memory in R/W mode from a
userspace process once it has been accessed by the kernel (it is then
cached by the QEMU TLB).

At the same time change the MMU_USER_IDX value so that it matches the
value used in recent kernels.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
