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/kernel/groups.c b/kernel/groups.c
index 6b2588d..90cf1c3 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -233,7 +233,7 @@
 	struct group_info *group_info;
 	int retval;
 
-	if (!nsown_capable(CAP_SETGID))
+	if (!ns_capable(current_user_ns(), CAP_SETGID))
 		return -EPERM;
 	if ((unsigned)gidsetsize > NGROUPS_MAX)
 		return -EINVAL;