Revert "capabitlies: ns_capable can use the cap helpers rather than lsm call"

This reverts commit d2a7009f.

J. R. Okajima explains:

 "After this commit, I am afraid access(2) on NFS may not work
  correctly.  The scenario based upon my guess.
   - access(2) overrides the credentials.
   - calls inode_permission() -- ... -- generic_permission() --
      ns_capable().
   - while the old ns_capable() calls security_capable(current_cred()),
     the new ns_capable() calls has_ns_capability(current) --
     security_capable(__task_cred(t)).

  current_cred() returns current->cred which is effective (overridden)
  credentials, but __task_cred(current) returns current->real_cred (the
  NFSD's credential).  And the overridden credentials by access(2) lost."
Requested-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/capability.c

@@ -384,7 +384,7 @@ bool ns_capable(struct user_namespace *ns, int cap)
 		BUG();
 	}
 
-	if (has_ns_capability(current, ns, cap)) {
+	if (security_capable(current_cred(), ns, cap) == 0) {
 		current->flags |= PF_SUPERPRIV;
 		return true;
 	}