selftests/rseq: Fix mm_cid test failure

Adapt the rseq.c/rseq.h code to follow GNU C library changes introduced by:

glibc commit 2e456ccf0c34 ("Linux: Make __rseq_size useful for feature detection (bug 31965)")

Without this fix, rseq selftests for mm_cid fail:

./run_param_test.sh
Default parameters
Running test spinlock
Running compare-twice test spinlock
Running mm_cid test spinlock
Error: cpu id getter unavailable

Fixes: 18c23558 ("selftests/rseq: Implement rseq mm_cid field support")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
CC: Carlos O'Donell <carlos@redhat.com>
CC: Florian Weimer <fweimer@redhat.com>
CC: linux-kselftest@vger.kernel.org
CC: stable@vger.kernel.org
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>