torture: Place all torture-test modules in one MAINTAINERS group

There is some confusion about where patches to kernel/torture.c
and kernel/locking/locktorture.c should be sent.  This commit
therefore updates MAINTAINERS appropriately.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>