ec: Fix some size_t vars to 64 bits even on 32 bits machines

The 64 bits 'trusted.ec.size' extended attribute was incorrectly
computed on 32 bits machines due to an overflow on negative
numbers.

Also changed some potentially dangerous uses of size_t in other
places.

This is a backport of http://review.gluster.org/8738/

Change-Id: Id76cfe49a2f350e564b5c71d8c8644fb9ce86662
BUG: 1144407
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8779
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>