libblkid: allow a lot of mac partitions

libblkid: allow a lot of mac partitions

If the map count is set to INT_MAX then the for loop does not stop
because its check is never false.

I have not found a correct upper limit. The other partition logics have
a maximum amount (exception is atari.c).

The loop itself wouldn't be endless. If the iteration reaches block 0
then the signature will be wrong. This means that map count = INT_MAX
case would fail even if such a setup would be correct on disk.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>