Revert "af_unix: fix hard linked sockets on overlay"

Revert "af_unix: fix hard linked sockets on overlay"

This reverts commit eb0a4a47ae89aaa0674ab3180de6a162f3be2ddf.

Since commit 51f7e52dc943 ("ovl: share inode for hard link") there's no
need to call d_real_inode() to check two overlay inodes for equality.

Side effect of this revert is that it's no longer possible to connect one
socket on overlayfs to one on the underlying layer (something which didn't
make sense anyway).

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