udp_tunnel: Use flex array to simplify code

'n_tables' is small, UDP_TUNNEL_NIC_MAX_TABLES	= 4 as a maximum. So there
is no real point to allocate the 'entries' pointers array with a dedicate
memory allocation.

Using a flexible array for struct udp_tunnel_nic->entries avoids the
overhead of an additional memory allocation.

This also saves an indirection when the array is accessed.

Finally, __counted_by() can be used for run-time bounds checking if
configured and supported by the compiler.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/4a096ba9cf981a588aa87235bb91e933ee162b3d.1695542544.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Paolo Abeni <pabeni@redhat.com>