powerpc/mce: Avoid nmi_enter/exit in real mode on pseries hash

Use of nmi_enter/exit in real mode handler causes the kernel to panic
and reboot on injecting SLB mutihit on pseries machine running in hash
MMU mode, because these calls try to accesses memory outside RMO
region in real mode handler where translation is disabled.

Add check to not to use these calls on pseries machine running in hash
MMU mode.

Fixes: 116ac378 ("powerpc/64s: machine check interrupt update NMI accounting")
Cc: stable@vger.kernel.org # v5.8+
Signed-off-by: Ganesh Goudar <ganeshgr@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201009064005.19777-2-ganeshgr@linux.ibm.com