bpf: Clean up sockmap related Kconfigs

As suggested by John, clean up sockmap related Kconfigs:

Reduce the scope of CONFIG_BPF_STREAM_PARSER down to TCP stream
parser, to reflect its name.

Make the rest sockmap code simply depend on CONFIG_BPF_SYSCALL
and CONFIG_INET, the latter is still needed at this point because
of TCP/UDP proto update. And leave CONFIG_NET_SOCK_MSG untouched,
as it is used by non-sockmap cases.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210223184934.6054-2-xiyou.wangcong@gmail.com
diff --git a/net/Kconfig b/net/Kconfig
index 8cea808..0ead7ec 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -317,13 +317,9 @@
 	select STREAM_PARSER
 	select NET_SOCK_MSG
 	help
-	  Enabling this allows a stream parser to be used with
+	  Enabling this allows a TCP stream parser to be used with
 	  BPF_MAP_TYPE_SOCKMAP.
 
-	  BPF_MAP_TYPE_SOCKMAP provides a map type to use with network sockets.
-	  It can be used to enforce socket policy, implement socket redirects,
-	  etc.
-
 config NET_FLOW_LIMIT
 	bool
 	depends on RPS