neighbour: fix neigh_dump_info() return value

Change neigh_dump_table() and pneigh_dump_table()
to either return 0 or -EMSGSIZE if not enough
space was available in the skb.

Then neigh_dump_info() can do the same.

This allows NLMSG_DONE to be appended to the current
skb at the end of a dump, saving a couple of recvmsg()
system calls.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>