batman-adv: split name from variable for uint mesh attributes

Some mesh attributes are behind substructs in the
batadv_priv object and for this reason the name cannot be
used anymore to refer to them.

This patch allows to specify the variable name where the
attribute is stored inside batadv_priv instead of using the
name

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>