powerpc/kernel: Print actual address of regs when oopsing

When we oops or otherwise call show_regs() we print the address of the
regs structure. Being able to see the address is fairly useful,
firstly to verify that the regs pointer is not completely bogus, and
secondly it allows you to dump the regs and surrounding memory with a
debugger if you have one.

In the normal case the regs will be located somewhere on the stack, so
printing their location discloses no further information than printing
the stack pointer does already.

So switch to %px and print the actual address, not the hashed value.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>