ovl: Provide a mount option metacopy=on/off for metadata copyup

By default metadata only copy up is disabled.  Provide a mount option so
that users can choose one way or other.

Also provide a kernel config and module option to enable/disable metacopy
feature.

metacopy feature requires redirect_dir=on when upper is present.
Otherwise, it requires redirect_dir=follow atleast.

As of now, metacopy does not work with nfs_export=on.  So if both
metacopy=on and nfs_export=on then nfs_export is disabled.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>