selftests/bpf: Use __NR_prlimit64 instead of __NR_getrlimit in user_ringbuf test

After commit 80d7da1c ("asm-generic: Drop getrlimit and setrlimit
syscalls from default list"), new architectures won't need to include
getrlimit and setrlimit, they are superseded with prlimit64.

In order to maintain compatibility for the new architectures, such as
LoongArch which does not define __NR_getrlimit, it is better to use
__NR_prlimit64 instead of __NR_getrlimit in user_ringbuf test to fix
the following build error:

    TEST-OBJ [test_progs] user_ringbuf.test.o
  tools/testing/selftests/bpf/prog_tests/user_ringbuf.c: In function 'kick_kernel_cb':
  tools/testing/selftests/bpf/prog_tests/user_ringbuf.c:593:17: error: '__NR_getrlimit' undeclared (first use in this function)
    593 |         syscall(__NR_getrlimit);
        |                 ^~~~~~~~~~~~~~
  tools/testing/selftests/bpf/prog_tests/user_ringbuf.c:593:17: note: each undeclared identifier is reported only once for each function it appears in
  make: *** [Makefile:573: tools/testing/selftests/bpf/user_ringbuf.test.o] Error 1
  make: Leaving directory 'tools/testing/selftests/bpf'

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1677235015-21717-4-git-send-email-yangtiezhu@loongson.cn