xfstests generic/306 fixes and updates

xfstests generic/306 fixes and updates

older xfs_io refused to write to /dev/null because it's
not a file on an xfs filesystem.  So add -F.

While we're at it, add more testcases:

* symlink on a RO fs pointing to a file on a RW fs.
* bind-mounted rw file on an RO fs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>