partitions: efi: tighten gpt header integrity checks

Hi Jens,

This is a resend of a patchset sent in early September to harden GPT header checks.

partitions: efi: check minimum header size
partitions: efi: verify header is outside usable area
partitions: efi: compare first and last usable LBAs

 block/partitions/efi.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

Thanks,
Davidlohr