ACPI: silence kmemcheck false positive

This addresses: https://bugzilla.kernel.org/show_bug.cgi?id=14998



We copy some strings into "event" but we leave the space after the NULL
terminators uninitialized.  Later in acpi_bus_receive_event() we copy
the whole struct to another buffer with memcpy().  If the new buffer is
stored on the stack, kmemcheck prints a warning about the unitialized
space after the NULL terminators.

It's true that the space is uninitialized, but it's harmless.  The
buffer is only used in acpi_system_read_event() and we don't read past
the NULL terminators.

This patch changes the kmalloc() to kzalloc() so that we initialize the
memory and silence the kmemcheck warning.

Reported-by: Christian Casteyde <casteyde.christian@free.fr>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>