net: Fix fall through warning in y2038 tstamp changes.

net/core/sock.c: In function 'sock_setsockopt':
net/core/sock.c:914:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   sock_set_flag(sk, SOCK_TSTAMP_NEW);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/sock.c:915:2: note: here
  case SO_TIMESTAMPING_OLD:
  ^~~~

Fixes: 9718475e ("socket: Add SO_TIMESTAMPING_NEW")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>