net: batman-adv: fix error handling

In the Linux kernel, the following vulnerability has been resolved:

net: batman-adv: fix error handling

Syzbot reported ODEBUG warning in batadv_nc_mesh_free(). The problem was
in wrong error handling in batadv_mesh_init().

Before this patch batadv_mesh_init() was calling batadv_mesh_free() in case
of any batadv_*_init() calls failure. This approach may work well, when
there is some kind of indicator, which can tell which parts of batadv are
initialized; but there isn't any.

All written above lead to cleaning up uninitialized fields. Even if we hide
ODEBUG warning by initializing bat_priv->nc.work, syzbot was able to hit
GPF in batadv_nc_purge_paths(), because hash pointer in still NULL. [1]

To fix these bugs we can unwind batadv_*_init() calls one by one.
It is good approach for 2 reasons: 1) It fixes bugs on error handling
path 2) It improves the performance, since we won't call unneeded
batadv_*_free() functions.

So, this patch makes all batadv_*_init() clean up all allocated memory
before returning with an error to no call correspoing batadv_*_free()
and open-codes batadv_mesh_free() with proper order to avoid touching
uninitialized fields.