iotests: Fix nonportable use of od --endian

iotests: Fix nonportable use of od --endian

Tests 261 and 272 fail on RHEL 7 with coreutils 8.22, since od
--endian was not added until coreutils 8.23.  Fix this by manually
constructing the final value one byte at a time.

Fixes: fc8ba423
Reported-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200226125424.481840-1-eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 69135eb30b9c3fca583737a96df015174dc8e6dd)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>