libceph: supported_features module parameter

Add a readonly, exported to sysfs module parameter so that userspace
can generate meaningful error messages.  It's a bit funky, but there is
no other libceph-specific place.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

net/ceph/ceph_common.c

@@ -45,6 +45,17 @@ bool libceph_compatible(void *data)
 }
 EXPORT_SYMBOL(libceph_compatible);
 
+static int param_get_supported_features(char *buffer,
+					const struct kernel_param *kp)
+{
+	return sprintf(buffer, "0x%llx", CEPH_FEATURES_SUPPORTED_DEFAULT);
+}
+static const struct kernel_param_ops param_ops_supported_features = {
+	.get = param_get_supported_features,
+};
+module_param_cb(supported_features, &param_ops_supported_features, NULL,
+		S_IRUGO);
+
 /*
  * find filename portion of a path (/foo/bar/baz -> baz)
  */