ovl: move some common code in a function

ovl_create_upper() and ovl_create_over_whiteout() seem to be sharing some
common code which can be moved into a separate function.  No functionality
change.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>