powerpc/pseries: PACA save area fix for general exception vs MCE

MCE must not use PACA_EXGEN. When a general exception enables MSR_RI,
that means SPRN_SRR[01] and SPRN_SPRG are no longer used. However the
PACA save area is still in use.
Acked-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>