net/core/filter: fix unused-variable warning

Building with CONFIG_INET=n will show the warning below:
net/core/filter.c: In function ‘____bpf_getsockopt’:
net/core/filter.c:4048:19: warning: unused variable ‘tp’ [-Wunused-variable]
  struct tcp_sock *tp;
                   ^~
net/core/filter.c:4046:31: warning: unused variable ‘icsk’ [-Wunused-variable]
  struct inet_connection_sock *icsk;
                               ^~~~
Move the variable declarations inside the {} block where they are used.

Fixes: 1e215300 ("bpf: add TCP_SAVE_SYN/TCP_SAVED_SYN options for bpf_(set|get)sockopt")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>