xfs/020: fix truncation test

xfs/020: fix truncation test

If we can't create the 60T sparse image for testing repair on a large fs
(such as when running on 32-bit), don't bother running the rest of the
test.  This requires the actual truncate(1) command, because it returns
nonzero if the system call fails.

[Eryu: the original bug was introduced by me when committing the
original patch, which was a correct fix. Sorry about that.. ]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>