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.