generic/{175,297,298}: fix use of uninitialized var

generic/{175,297,298}: fix use of uninitialized var

The truncate command in those tests use an uninitialized variable i.
in kdevops, i must contain some leftover, so we get errors like:
/data/fstests-install/xfstests/tests/generic/298: line 45: /dev/loop12):
           syntax error: operand expected (error token is "/dev/loop12)")

Apparently, noone including the author of the tests knows why this
truncate command is in the test, so remove the wrong truncate command.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>