selftests/tls: Add {} to avoid static checker warning

This silences a static checker warning due to the unusual macro
construction of EXPECT_*() by adding explicit {}s around the enclosing
while loop.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 7f657d5b ("selftests: tls: add selftests for TLS sockets")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>