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 <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211006185619.364369-13-fallentree@fb.com