[BZ #838]

[BZ #838]

	* malloc/arena.c (ptmalloc_lock_all): If global lock already taken
	by the same thread, just bump the counter.
	(ptmalloc_unlock_all): If counter for recursive locks hasn't reached
	zero, don't do anything else.
	* malloc/Makefile (tests): Add tst-mallocfork.
	* malloc/tst-mallocfork.c: New file.