selftests/bpf: make fexit_stress test run in serial mode

fexit_stress is attaching maximum allowed amount of fexit programs to
bpf_fentry_test1 kernel function, which is used by a bunch of other
parallel tests, thus pretty frequently interfering with their execution.

Given the test assumes nothing else is attaching to bpf_fentry_test1,
mark it serial.
Suggested-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220511232012.609370-1-andrii@kernel.orgSigned-off-by: NAlexei Starovoitov <ast@kernel.org>

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

@@ -5,7 +5,7 @@
 /* that's kernel internal BPF_MAX_TRAMP_PROGS define */
 #define CNT 38
 
-void test_fexit_stress(void)
+void serial_test_fexit_stress(void)
 {
 	char test_skb[128] = {};
 	int fexit_fd[CNT] = {};