loader.efi: finally fix the off-by-one bug in efi_verify_staging_size()

r314828(loader.efi: fix an off-by-one bug in efi_verify_staging_size())
doesn't really fix the bug and this patch adds the missing part.

It's a shame that I didn't make everything correct at the very beginning...

MFC after: 2 weeks
Sponsored by: Microsoft