UBUNTU: SAUCE: efi: efi_get_memory_map -- increase map headroom

UBUNTU: SAUCE: efi: efi_get_memory_map -- increase map headroom

We are seeing some EFI based machines failing to boot hard in the EFI
stub:

    exit_boot() failed!
    efi_main() failed!

This seems to occur when the bootloader (grub2 in this case) has had
to manipulate some additional files due to a change in the way MAAS
boots the machines.  We tracked this down to the memory map dance
efi_get_memory_map().  Basically we attempt to close boot services and
it informs us it cannot do so because it failed to record the updated
memory map.  This occurs when there is insufficient space in the passed
memory map buffer to record changes during the operation.  At the point
when this occurs we are unable to call the allocation functions to
reallocate the buffer so we panic.

To avoid this we allocate some additional entries in the buffer to cover
any additional entries.  This headroom is currently insufficient for
these machines under this use case.  Increase EFI_MMAP_NR_SLACK_SLOTS to
provide space for more memory map modifications.

BugLink: https://bugs.launchpad.net/bugs/1851810
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>