xfs/229: do not _xfs_force_bdev on TEST_DIR

xfs/229: do not _xfs_force_bdev on TEST_DIR

In commit ea15099b71, I observed that this test tries to test the
behavior of the extent size hint on the data device.  If the test runner
set up MKFS_OPTIONS such that the filesystem gets created with a
realtime section and rtinherit set on the root directory, then the
preconditions of this test (creating files on the data section) is not
satisfied and the results of this test are incorrect.  The solution was
to force all files created by this test to be assigned to the data
section.

Unfortunately, the correction that I made has side effects beyond this
test -- by clearing rtinherit on $TEST_DIR, all tests that run after
this one will create files on the data section, because the test
filesystem persists for the duration of the entire test run.  This leads
to the wrong things being tested.

Fix this new problem by clearing the rtinherit flag on $TDIR, which
contains the files created by this test and is removed during cleanup,
and leave a few comments celebrating our new discoveries.

Fixes: ea15099b71 ("xfs: force file creation to the data device for certain layout tests")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>