MFC r258078,258079

Increase the stack size for ppc64 from 4 pages to 8.

I found a stack overflow when a coredump was taken onto a ZFS volume with
heavy network activity.  2 DSI traps, plus one DECR trap, along with several
function calls in the stack, overflowed the 4 pages.  8 page stack fixes this.

Discussed with: nwhitehorn
Approved by:	re
Relnotes:	yes

sys/powerpc/include/param.h

@@ -104,8 +104,12 @@
 #define	MAXPAGESIZES	1		/* maximum number of supported page sizes */
 
 #ifndef KSTACK_PAGES
+#ifdef __powerpc64__
+#define	KSTACK_PAGES		8		/* includes pcb */
+#else
 #define	KSTACK_PAGES		4		/* includes pcb */
 #endif
+#endif
 #define	KSTACK_GUARD_PAGES	1	/* pages of kstack guard; 0 disables */
 #define	USPACE		(KSTACK_PAGES * PAGE_SIZE)	/* total size of pcb */