neigh: some entries can be skipped during dumping

neightbl_dump_info and neigh_dump_table  can skip entries if the
*fill*info functions return an error. This results in an incomplete
dump ((invoked by netlink requests for RTM_GETNEIGHTBL or
RTM_GETNEIGH)

nidx and idx should not be incremented if the current entry was not
placed in the output buffer

Signed-off-by: Gautam Kachroo <gk@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>