vfs: make argument of d_real_inode() const

d_op->d_real() leaves the dentry alone except if the third argument is
non-zero.  Unfortunately very difficult to explain to the compiler without
a cast.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Acked-by: Jeff Layton <jlayton@poochiereds.net>