powerpc: Always initialize input array when calling epapr_hypercall()

Several callers to epapr_hypercall() pass an uninitialized stack
allocated array for the input arguments, presumably because they
have no input arguments. However this can produce errors like
this one

 arch/powerpc/include/asm/epapr_hcalls.h:470:42: error: 'in' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  unsigned long register r3 asm("r3") = in[0];
                                        ~~^~~

Fix callers to this function to always zero-initialize the input
arguments array to prevent this.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>