ceph: Only allow mounts in the initial network namespace

Today ceph opens tcp sockets from a delayed work callback.  Delayed
work happens from kernel threads which are always in the initial
network namespace.   Therefore fail early if someone attempts
to mount a ceph filesystem from something other than the initial
network namespace.

Cc: Sage Weil <sage@inktank.com>
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

net/ceph/ceph_common.c

@@ -15,6 +15,8 @@
 #include <linux/slab.h>
 #include <linux/statfs.h>
 #include <linux/string.h>
+#include <linux/nsproxy.h>
+#include <net/net_namespace.h>
 
 
 #include <linux/ceph/ceph_features.h>
@@ -292,6 +294,9 @@ ceph_parse_options(char *options, const char *dev_name,
 	int err = -ENOMEM;
 	substring_t argstr[MAX_OPT_ARGS];
 
+	if (current->nsproxy->net_ns != &init_net)
+		return ERR_PTR(-EINVAL);
+
 	opt = kzalloc(sizeof(*opt), GFP_KERNEL);
 	if (!opt)
 		return ERR_PTR(-ENOMEM);