net: tipc: Fix parameter types passed to %s formater

Now that the compiler is performing printf checking, we get the warning:

net/tipc/netlink_compat.c: In function ‘tipc_nl_compat_link_stat_dump’:
net/tipc/netlink_compat.c:591:39: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘void *’ [-Wformat=]
  591 |  tipc_tlv_sprintf(msg->rep, "\nLink <%s>\n",
      |                                      ~^
      |                                       |
      |                                       char *
      |                                      %p
  592 |     nla_data(link[TIPC_NLA_LINK_NAME]));
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |     |
      |     void *

There is no nla_string(), so cast to a char *.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201028004333.929816-1-andrew@lunn.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>