rtnetlink: change nlk->cb_mutex role

In commit af65bdfc ("[NETLINK]: Switch cb_lock spinlock
to mutex and allow to override it"), Patrick McHardy used
a common mutex to protect both nlk->cb and the dump() operations.

The override is used for rtnl dumps, registered with
rntl_register() and rntl_register_module().

We want to be able to opt-out some dump() operations
to not acquire RTNL, so we need to protect nlk->cb
with a per socket mutex.

This patch renames nlk->cb_def_mutex to nlk->nl_cb_mutex

The optional pointer to the mutex used to protect dump()
call is stored in nlk->dump_cb_mutex

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>