powerpc/book3e: Fix extlb size

The calculation of the size for the exception save area of the TLB
miss handler is wrong, luckily it's too big not too small.

Rework it to make it a bit clearer, and also correct. We want 3 save
areas, each EX_TLB_SIZE _bytes_.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/paca.h

@@ -106,7 +106,8 @@ struct paca_struct {
 	pgd_t *pgd;			/* Current PGD */
 	pgd_t *kernel_pgd;		/* Kernel PGD */
 	u64 exgen[8] __attribute__((aligned(0x80)));
-	u64 extlb[EX_TLB_SIZE*3] __attribute__((aligned(0x80)));
+	/* We can have up to 3 levels of reentrancy in the TLB miss handler */
+	u64 extlb[3][EX_TLB_SIZE / sizeof(u64)] __attribute__((aligned(0x80)));
 	u64 exmc[8];		/* used for machine checks */
 	u64 excrit[8];		/* used for crit interrupts */
 	u64 exdbg[8];		/* used for debug interrupts */