Clear list of acquired robust mutexes in the child process after forking.

Clear list of acquired robust mutexes in the child process after forking.

Robust mutexes acquired at the time of a call to fork() do not remain
acquired by the forked child process.  We have to clear the list of
acquired robust mutexes before registering this list with the kernel;
otherwise, if some of the robust mutexes are process-shared, the parent
process can alter the child's robust mutex list, which can lead to
deadlocks or even modification of memory that may not be occupied by a
mutex anymore.

	[BZ #19402]
	* sysdeps/nptl/fork.c (__libc_fork): Clear list of acquired robust
	mutexes.