generic/388: use the src/godown program instead of xfs_io

generic/388: use the src/godown program instead of xfs_io

The "xfs_io -xc shutdown $MNT" command doesn't work on non-xfs file
systems.  So use the equivalent src/godown program instead, which is
how the other tests which use the shutdown command were converted from
xfs-specific tests to generic tests.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>