net: ping: fix constness inconsistency in ipv6_chk_addr

The ipv6_chk_addr declarations and implementations aren't consistent
about the dev parameter's constness.  6bc19fb upstream resolved this in
favor of const, so let's do the same here.

This fixes the following warning:

net/ipv6/ping.c: In function 'pingv6_init':
net/ipv6/ping.c:87:27: warning: assignment from incompatible pointer type
  pingv6_ops.ipv6_chk_addr = ipv6_chk_addr;
                           ^

Change-Id: Ibd9efd7d489e661f71ab641c981689bf92258b7c
Signed-off-by: Greg Hackmann <ghackmann@google.com>