rwlock: Fix explicit hand-over (bug 21298)

Without this fix, the rwlock can fail to execute the explicit hand-over
in certain cases (e.g., empty critical sections that switch quickly between
read and write phases).  This can then lead to errors in how __wrphase_futex
is accessed, which in turn can lead to deadlocks.

support/xpthread_rwlock_rdlock.c

@@ -0,0 +1,26 @@
+/* pthread_rwlock_rdlock with error checking.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+
+void
+xpthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
+{
+  xpthread_check_return ("pthread_rwlock_rdlock",
+			 pthread_rwlock_rdlock (rwlock));
+}