ovl: lockdep annotate of nested OVL_I(inode)->lock

This fixes a lockdep splat when mounting a nested overlayfs.

Fixes: a015dafc ("ovl: use ovl_inode mutex to synchronize...")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>