Use truncate instead of fallocate in ziltest.sh

Use truncate instead of fallocate in ziltest.sh

For the purposes of creating sparse files the truncate command is
preferable to fallocate because generic sparse files are more widely
supported by older platforms.  Specifically Debian Wheezy which is
based on a 2.6.32 kernel used ext3 by default which at the time did
not support it.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>