selftests: bpf: fix use of undeclared RET_IF macro

commit 634efb75 ("selftests: bpf: Reset global state between
reuseport test runs") uses a macro RET_IF which doesn't exist in
the v4.19 tree. It is defined as follows:

        #define RET_IF(condition, tag, format...) ({
                if (CHECK_FAIL(condition)) {
                        printf(tag " " format);
                        return;
                }
        })

CHECK_FAIL in turn is defined as:

        #define CHECK_FAIL(condition) ({
                int __ret = !!(condition);
                int __save_errno = errno;
                if (__ret) {
                        test__fail();
                        fprintf(stdout, "%s:FAIL:%d\n", __func__, __LINE__);
                }
                errno = __save_errno;
                __ret;
        })

Replace occurences of RET_IF with CHECK. This will abort the test binary
if clearing the intermediate state fails.

Fixes: 634efb75 ("selftests: bpf: Reset global state between reuseport test runs")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>