selftests/bpf: Fix pid check in fexit_sleep test

bpf_get_current_pid_tgid() returns u64, whose upper 32 bits are the same
as userspace getpid() return value.

Signed-off-by: Yucong Sun <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/fexit_sleep.c

···
 SEC("fentry/__x64_sys_nanosleep")
 int BPF_PROG(nanosleep_fentry, const struct pt_regs *regs)
 {
-	if ((int)bpf_get_current_pid_tgid() != pid)
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
 		return 0;
 
 	fentry_cnt++;
···
 SEC("fexit/__x64_sys_nanosleep")
 int BPF_PROG(nanosleep_fexit, const struct pt_regs *regs, int ret)
 {
-	if ((int)bpf_get_current_pid_tgid() != pid)
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
 		return 0;
 
 	fexit_cnt++;