include/nuttx/net and net/: Move private prototype of netdev_foreach() from the public include/nuttx/net/net.h to the private net/netdev/ndetdev.h header file where it belongs.

net/devif/ipv6_input.c

@@ -87,7 +87,6 @@
 
 #include <net/if.h>
 
-#include <nuttx/net/net.h>
 #include <nuttx/net/netconfig.h>
 #include <nuttx/net/netdev.h>
 #include <nuttx/net/netstats.h>
@@ -242,6 +241,7 @@ static bool check_destipaddr(FAR struct net_driver_s *dev,
  *   to this input.
  *
  * Assumptions:
+ *   The network is locked.
  *
  ****************************************************************************/