Tests: changing calls from 'fstat' to 'fstat64' fixes these tests on 32 bit platforms: bug-858242.t, bug-808400*.t

Change-Id: Ifd85c711a8d16eb3ef17bd1c585acdc34121b12d
BUG: 962226
Signed-off-by: John Smith <lbalbalba@gmail.com>
Reviewed-on: http://review.gluster.org/5034
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>