inet: fix compilation warnings in fqdir_pre_exit()

inet: fix compilation warnings in fqdir_pre_exit()

The linux-next commit "inet: fix various use-after-free in defrags
units" [1] introduced compilation warnings,

./include/net/inet_frag.h:117:1: warning: 'inline' is not at beginning
of declaration [-Wold-style-declaration]
 static void inline fqdir_pre_exit(struct fqdir *fqdir)
 ^~~~~~
In file included from ./include/net/netns/ipv4.h:10,
                 from ./include/net/net_namespace.h:20,
                 from ./include/linux/netdevice.h:38,
                 from ./include/linux/icmpv6.h:13,
                 from ./include/linux/ipv6.h:86,
                 from ./include/net/ipv6.h:12,
                 from ./include/rdma/ib_verbs.h:51,
                 from ./include/linux/mlx5/device.h:37,
                 from ./include/linux/mlx5/driver.h:51,
                 from
drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c:37:

[1] https://lore.kernel.org/netdev/20190618180900.88939-3-edumazet@google.com/

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: David S. Miller <davem@davemloft.net>