ovl: create directories inside merged parent opaque

The benefit of making directories opaque on creation is that lookups can
stop short when they reach the original created directory, instead of
continue lookup the entire depth of parent directory stack.

The best case is overlay with N layers, performing lookup for first level
directory, which exists only in upper.  In that case, there will be only
one lookup instead of N.

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