bpf: add missing break in for the TCP_BPF_SNDCWND_CLAMP case

There appears to be a missing break in the TCP_BPF_SNDCWND_CLAMP case.
Currently the non-error path where val is greater than zero falls through
to the default case that sets the error return to -EINVAL. Add in
the missing break.

Detected by CoverityScan, CID#1449376 ("Missing break in switch")

Fixes: 13bf9641 ("bpf: Adds support for setting sndcwnd clamp")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>