drbd: Also need to check for DRBD_GENLA_F_MANDATORY flags before nla_find_nested()

This is done by introducing drbd_nla_find_nested() which handles the flag
before calling nla_find_nested().

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
diff --git a/include/linux/drbd_genl.h b/include/linux/drbd_genl.h
index 47ef324..0c2102c 100644
--- a/include/linux/drbd_genl.h
+++ b/include/linux/drbd_genl.h
@@ -96,7 +96,7 @@
  * and the volume id within the resource. */
 GENL_struct(DRBD_NLA_CFG_CONTEXT, 2, drbd_cfg_context,
 	__u32_field(1, DRBD_GENLA_F_MANDATORY,	ctx_volume)
-	__str_field(2, DRBD_GENLA_F_MANDATORY,	ctx_conn_name, 128)
+	__str_field(2, DRBD_GENLA_F_MANDATORY,	ctx_resource_name, 128)
 )
 
 GENL_struct(DRBD_NLA_DISK_CONF, 3, disk_conf,