[JFFS2] Reinstate NFS exportability

Now that the readdir/lookup deadlock issues have been dealt with, we can
export JFFS2 file systems again.

(For now, you have to specify fsid manually; we should add a method to
the export_ops to handle that too.)

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>