Consider increasing the number of `EXTRA_ENTRIES` allocated for the exit_boot_services memory map

[nicholasbishop]

See this linux efi patch: https://marc.info/?l=linux-efi&m=173376826126650&w=2

Our definition is here:

uefi-rs/uefi/src/mem/memory_map/impl_.rs

Line 323 in c2d15a8

const EXTRA_ENTRIES: usize = 8;