efi_loader: correct efi_add_known_memory()

If a memory bank is not EFI_PAGE_SIZE aligned efi_add_known_memory() the
number of memory pages may be incorrectly calculated.

We have to round up the start address and to round down the end address
to determine which complete pages are provided by the memory bank.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>