net: Fix continued iteration in rtnl_bridge_getlink()

Commit e5a55a89 ('net: create generic
bridge ops') broke the handling of a non-zero starting index in
rtnl_bridge_getlink() (based on the old br_dump_ifinfo()).

When the starting index is non-zero, we need to increment the current
index for each entry that we are skipping.  Also, we need to check the
index before both cases, since we may previously have stopped
iteration between getting information about a device from its master
and from itself.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Tested-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>