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>

arch/powerpc/kernel/exceptions-64s.S

@@ -969,14 +969,14 @@ ALT_MMU_FTR_SECTION_END_IFCLR(MMU_FTR_TYPE_RADIX)
 machine_check_common:
 
 	mfspr	r10,SPRN_DAR
-	std	r10,PACA_EXGEN+EX_DAR(r13)
+	std	r10,PACA_EXMC+EX_DAR(r13)
 	mfspr	r10,SPRN_DSISR
-	stw	r10,PACA_EXGEN+EX_DSISR(r13)
+	stw	r10,PACA_EXMC+EX_DSISR(r13)
 	EXCEPTION_PROLOG_COMMON(0x200, PACA_EXMC)
 	FINISH_NAP
 	RECONCILE_IRQ_STATE(r10, r11)
-	ld	r3,PACA_EXGEN+EX_DAR(r13)
-	lwz	r4,PACA_EXGEN+EX_DSISR(r13)
+	ld	r3,PACA_EXMC+EX_DAR(r13)
+	lwz	r4,PACA_EXMC+EX_DSISR(r13)
 	std	r3,_DAR(r1)
 	std	r4,_DSISR(r1)
 	bl	save_nvgprs