libblkid: use BLKSSZGET for GPT sectors

libblkid: use BLKSSZGET for GPT sectors

The current implementation uses fixed sector size (512 bytes), that's
wrong. UEFI standard requires real logical sector size -- it means
BLKSSZGET for Linux.

The size of GPT header is not static, but whole sector is allocated
for the header. In theory the HeaderSize field could be greater than
sizeof(struct gpt_header). It means we have to read whole sector with
the header, because the header crc32 checksum is counted according to
HeaderSize.

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