xfs: test for post umount readahead completion panic

xfs: test for post umount readahead completion panic

XFS has a bug where directory readahead completions can occur after
unmount. This can lead to a crash or panic because metadata read
verification attempts to access core XFS data structures (e.g., the
log) after they have been freed and certain pointers have been
reset.

Add a test that triggers directory readahead, delays the readahead
I/O and immediately unmounts the filesystem. This test is part of
the dangerous group as it will cause kernels affected by the bug to
crash.

[eguan replaced touch with echo to speedup file creation]

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>