batman-adv: create hardif only for netdevs that are part of a mesh

batman-adv is using netdev notifiers to create a hard_iface struct for
every Ethernet-like netdev in the system. These hardifs are tracked in a
global linked list, which results in a few performance issues:

Lookups in this list are O(n) in the total number of netdevs. As a
hardif is looked up when a netdev is removed, this also takes O(n) in
the number of netdevs, and removing n netdevs may take O(n^2). This
slowdown will always happen when the batman-adv module is loaded, no
mesh needs to be active.

With the hardif being referenced as iflink private data, the global list
is only needed for hardifs that are *not* part of a mesh (that is, the
hardif is unused). To prepare for removing the global list, only create
a hardif struct when an interface is added to a mesh and destroy it on
removal.

As adding/removing and enabling/disabling a hardif become one and the
same, batadv_hardif_add_interface() is merged into
batadv_hardif_enable_interface(), and batadv_hardif_remove_interface()
can be dropped altogether.

Signed-off-by: Nora Schiffer <neocturne@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>

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