libfdisk: fix use of on-disk sizeof_partition_entry in GPT

libfdisk: fix use of on-disk sizeof_partition_entry in GPT

The GPT code hardcodes sizeof(struct gpt_entry) (128 bytes) in
several places instead of using the on-disk sizeof_partition_entry
field from the GPT header. The UEFI specification allows entry sizes
larger than 128 bytes.

 - reject GPT headers where sizeof_partition_entry is smaller than
   sizeof(struct gpt_entry) to prevent out-of-bounds reads when
   accessing entry array elements

 - use the on-disk entry size as the qsort element stride in
   gpt_reorder() to avoid corrupting entries when sizeof_partition_entry
   differs from 128

 - use the on-disk entry size in gpt_copy_header() when calculating
   the backup partition entry array LBA

Signed-off-by: Karel Zak <kzak@redhat.com>