neigh: make struct neigh_table::entry_size unsigned int

Neigh entry size can't be negative.

Space savings:

	add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-7 (-7)
	function                                     old     new   delta
	lowpan_neigh_construct                        25      24      -1
	clip_seq_sub_iter                            152     151      -1
	clip_ioctl                                  1475    1474      -1
	clip_constructor                              93      92      -1
	__neigh_create                              2455    2452      -3

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -190,7 +190,7 @@ struct neigh_hash_table {
 
 struct neigh_table {
 	int			family;
-	int			entry_size;
+	unsigned int		entry_size;
 	int			key_len;
 	__be16			protocol;
 	__u32			(*hash)(const void *pkey,