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: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 /* 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] = {};