userns:  Kill nsown_capable it makes the wrong thing easy

nsown_capable is a special case of ns_capable essentially for just CAP_SETUID and
CAP_SETGID.  For the existing users it doesn't noticably simplify things and
from the suggested patches I have seen it encourages people to do the wrong
thing.  So remove nsown_capable.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index f9765203..81d3a9a 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -651,7 +651,7 @@
 	struct net *net = ns;
 
 	if (!ns_capable(net->user_ns, CAP_SYS_ADMIN) ||
-	    !nsown_capable(CAP_SYS_ADMIN))
+	    !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
 		return -EPERM;
 
 	put_net(nsproxy->net_ns);