ACPI: buffer array too short in drivers/acpi/system.c

Since "ff_gbl_lock" has a length of 11 chars and is copied with sprintf
to char buffer[10], there is a problem. We need char buffer[12] because
of the closing zero byte.

Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
Signed-off-by: Len Brown <len.brown@intel.com>