anon_inodefs: forbid open via /proc

anon_inodefs: forbid open via /proc

open("/proc/pid/$anon-fd") should fail, we can't create the new file with
correct f_op/etc correctly.  Currently this creates the bogus file with
the empty anon_inode_fops, this is harmless but still wrong and
misleading.

Add anon_inode_fops->anon_open() which simply returns ENXIO like
sock_no_open() does in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>