rtnetlink: Protect struct rtnl_af_ops with SRCU.

Once RTNL is replaced with rtnl_net_lock(), we need a mechanism to
guarantee that rtnl_af_ops is alive during inflight RTM_SETLINK
even when its module is being unloaded.

Let's use SRCU to protect ops.

rtnl_af_lookup() now iterates rtnl_af_ops under RCU and returns
SRCU-protected ops pointer.  The caller must call rtnl_af_put()
to release the pointer after the use.

Also, rtnl_af_unregister() unlinks the ops first and calls
synchronize_srcu() to wait for inflight RTM_SETLINK requests to
complete.

Note that rtnl_af_ops needs to be protected by its dedicated lock
when RTNL is removed.

Note also that BUG_ON() in do_setlink() is changed to the normal
error handling as a different af_ops might be found after
validate_linkmsg().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>