ovl: constant d_ino across copy up

When all layers are on the same fs, and iterating a directory which may
contain copy up entries, call vfs_getattr() on the overlay entries to make
sure that d_ino will be consistent with st_ino from stat(2).

There is an overhead of lookup per upper entry in readdir.

The overhead is minimal if the iterated entries are already in dcache.  It
is also quite useful for the common case of 'ls -l' that readdir() pre
populates the dcache with the listed entries, making the following stat()
calls faster.

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