net: socket: Fix missing break in switch statement

Add missing break statement in order to prevent the code from falling
through to cases SIOCGSTAMP_NEW and SIOCGSTAMPNS_NEW.

This bug was found thanks to the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: 0768e170 ("net: socket: implement 64-bit timestamps")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>