Improvements to GPT reading code.

Improvements to GPT reading code.

looking at the gpt code in util.c i found i did not like it at all, a
gpt partition entry is currently 128 bytes, but the spec does not say it
is a fixed value, so the code that reads into a buffer with 512bytes
chunk expecting this to be a multiplier of part_size is imho incorrect.
my fix was to read each partition entry directly into a struct
GPT_part_entry, the advantage is that the code is very simple to read,
the disadvantage it is 128 reads of 128 bytes each, which is
sub-optimal, but i believe readahead will mitigate this a lot.

Signed-off-by: NeilBrown <neilb@suse.de>