Unlock the malloc() locks in the child process after fork(). This gives

us working malloc in the fork child of the multithreaded process.

Although POSIX requires that only async-signal safe functions shall be
operable after fork in multithreaded process, not having malloc lower
the quality of our implementation.

Tested by:	rink
Discussed with:	kan, davidxu
Reviewed by:	kan
MFC after:	1 month

lib/libthr/thread/thr_fork.c

@@ -172,6 +172,7 @@ _fork(void)
 
 		if (unlock_malloc) {
 			_rtld_atfork_post(rtld_locks);
+			_malloc_postfork();
 		}
 
 		/* Run down atfork child handlers. */