x86, efi: retry ExitBootServices() on failure

ExitBootServices is absolutely supposed to return a failure if any
ExitBootServices event handler changes the memory map.  Basically the
get_map loop should run again if ExitBootServices returns an error the
first time.  I would say it would be fair that if ExitBootServices gives
an error the second time then Linux would be fine in returning control
back to BIOS.

The second change is the following line:

again:
        size += sizeof(*mem_map) * 2;

Originally you were incrementing it by the size of one memory map entry.
The issue here is all related to the low_alloc routine you are using.
In this routine you are making allocations to get the memory map itself.
Doing this allocation or allocations can affect the memory map by more
than one record.

[ mfleming - changelog, code style ]
Signed-off-by: Zach Bobroff <zacharyb@ami.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>