xen/x86/vpmu: Zero struct pt_regs before calling into sample handling code

commit 70513d58751d7c6c1a0133557b13089b9f2e3e66 upstream.

Otherwise we may leak kernel stack for events that sample user
registers.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>