efi: fix allocation problems if ExitBootServices() fails

efi: fix allocation problems if ExitBootServices() fails

If calling ExitBootServices() fails, the required memory map size may
have increased. When initially allocating the memory map, allocate a
slightly larger buffer (by an arbitrary 8 entries) to fix this.

The ARM code path was already allocating a larger buffer than required,
so this moves the code to be common for all architectures.

This was seen on the following machine when using the iscsidxe UEFI
driver. The machine would consistently fail the first call to
ExitBootServices().
System Information
        Manufacturer: Supermicro
        Product Name: X10SLE-F/HF
BIOS Information
        Vendor: American Megatrends Inc.
        Version: 2.00
        Release Date: 04/24/2014

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>