Revert "af_unix: fix hard linked sockets on overlay"

Revert "af_unix: fix hard linked sockets on overlay"

This reverts commit b2d24e450488b51901d5c2b8f9d717a68d8c049f.

Since commit da6ea55c7844 ("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>