selftests/bpf: Add traffic monitor functions.

Add functions that capture packets and print log in the background. They
are supposed to be used for debugging flaky network test cases. A monitored
test case should call traffic_monitor_start() to start a thread to capture
packets in the background for a given namespace and call
traffic_monitor_stop() to stop capturing. (Or, option '-m' implemented by
the later patches.)

    lo      In  IPv4 127.0.0.1:40265 > 127.0.0.1:55907: TCP, length 68, SYN
    lo      In  IPv4 127.0.0.1:55907 > 127.0.0.1:40265: TCP, length 60, SYN, ACK
    lo      In  IPv4 127.0.0.1:40265 > 127.0.0.1:55907: TCP, length 60, ACK
    lo      In  IPv4 127.0.0.1:55907 > 127.0.0.1:40265: TCP, length 52, ACK
    lo      In  IPv4 127.0.0.1:40265 > 127.0.0.1:55907: TCP, length 52, FIN, ACK
    lo      In  IPv4 127.0.0.1:55907 > 127.0.0.1:40265: TCP, length 52, RST, ACK
    Packet file: packets-2173-86-select_reuseport:sockhash_IPv4_TCP_LOOPBACK_test_detach_bpf-test.log
    torvalds#280/87 select_reuseport/sockhash IPv4/TCP LOOPBACK test_detach_bpf:OK

The above is the output of an example. It shows the packets of a connection
and the name of the file that contains captured packets in the directory
/tmp/tmon_pcap. The file can be loaded by tcpdump or wireshark.

This feature only works if libpcap is available. (Could be found by pkg-config)

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>

tools/testing/selftests/bpf/Makefile

@@ -41,6 +41,10 @@ CFLAGS += -g $(OPT_FLAGS) -rdynamic					\
 LDFLAGS += $(SAN_LDFLAGS)
 LDLIBS += $(LIBELF_LIBS) -lz -lrt -lpthread
 
+LDLIBS += $(shell $(PKG_CONFIG) --libs libpcap 2>/dev/null)
+CFLAGS += $(shell $(PKG_CONFIG) --cflags libpcap 2>/dev/null)
+CFLAGS += $(shell $(PKG_CONFIG) --exists libpcap 2>/dev/null && echo "-DTRAFFIC_MONITOR=1")
+
 # The following tests perform type punning and they may break strict
 # aliasing rules, which are exploited by both GCC and clang by default
 # while optimizing.  This can lead to broken programs.