selftests/bpf: Initialize duration variable before using

The 'duration' variable is referenced in the CHECK() macro, and there are
some uses of the macro before 'duration' is set.  The clang compiler
(validly) complains about this.

Sample error:

.../selftests/bpf/prog_tests/fexit_test.c:23:6: warning: variable 'duration' is uninitialized when used here [-Wuninitialized]
        if (CHECK(err, "prog_load sched cls", "err %d errno %d\n", err, errno))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../selftests/bpf/test_progs.h:134:25: note: expanded from macro 'CHECK'
        if (CHECK(err, "prog_load sched cls", "err %d errno %d\n", err, errno))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        _CHECK(condition, tag, duration, format)
                               ^~~~~~~~

Signed-off-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200123235144.93610-1-sdf@google.com

tools/testing/selftests/bpf/prog_tests/fentry_test.c

@@ -9,7 +9,7 @@ void test_fentry_test(void)
 	struct test_pkt_access *pkt_skel = NULL;
 	struct fentry_test *fentry_skel = NULL;
 	int err, pkt_fd, i;
-	__u32 duration, retval;
+	__u32 duration = 0, retval;
 	__u64 *result;
 
 	pkt_skel = test_pkt_access__open_and_load();

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

@@ -11,7 +11,7 @@ static void test_fexit_bpf2bpf_common(const char *obj_file,
 	int err, pkt_fd, i;
 	struct bpf_link **link = NULL;
 	struct bpf_program **prog = NULL;
-	__u32 duration, retval;
+	__u32 duration = 0, retval;
 	struct bpf_map *data_map;
 	const int zero = 0;
 	u64 *result = NULL;

tools/testing/selftests/bpf/prog_tests/fexit_test.c

@@ -13,7 +13,7 @@ void test_fexit_test(void)
 	int err, pkt_fd, kfree_skb_fd, i;
 	struct bpf_link *link[6] = {};
 	struct bpf_program *prog[6];
-	__u32 duration, retval;
+	__u32 duration = 0, retval;
 	struct bpf_map *data_map;
 	const int zero = 0;
 	u64 result[6];