powerpc/xmon: Add kstack base to paca dump

When dumping the paca in xmon we currently show kstack. Although it's
not hard it's a bit fiddly to work out what the bounds of the kernel
stack should be based on the kstack value.

To make life easier and "kstack_base" which is the base (lowest
address) of the kernel stack, eg:

 kstack               = 0xc0000000f1a7be30      (0x258)
 kstack_base          = 0xc0000000f1a78000
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/xmon/xmon.c

@@ -2384,6 +2384,7 @@ static void dump_one_paca(int cpu)
 #endif
 	DUMP(p, __current, "p");
 	DUMP(p, kstack, "lx");
+	printf(" kstack_base          = 0x%016lx\n", p->kstack & ~(THREAD_SIZE - 1));
 	DUMP(p, stab_rr, "lx");
 	DUMP(p, saved_r1, "lx");
 	DUMP(p, trap_save, "x");