Fix assert_havelock(::ReentrantLock) to assert that the _current-ta…

…sk_ has the lock.

Before this commit, new threads would incorrectly believe that they held
a lock on a Condition when they actually didn't, and would allow illegal
operations, e.g. notify:

```julia
julia> c = Threads.Condition()
Base.GenericCondition{ReentrantLock}(Base.InvasiveLinkedList{Task}(nothing, nothing), ReentrantLock(nothing, Base.GenericCondition{Base.Threads.SpinLock}(Base.InvasiveLinkedList{Task}(nothing, nothing), Base.Threads.SpinLock(Base.Threads.Atomic{Int64}(0))), 0))

julia> lock(c)

julia> fetch(Threads.@Spawn Base.assert_havelock(c))  # This should be an ERROR (the new thread doesn't have the lock)

julia> fetch(Threads.@Spawn notify(c))                # This should be an ERROR (the new thread doesn't have the lock)
0

julia> fetch(Threads.@Spawn wait(c))                  # This error should be caught earlier (in assert_havelock).
ERROR: TaskFailedException:
unlock from wrong thread
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] unlockall(::ReentrantLock) at ./lock.jl:121
 [3] wait(::Base.GenericCondition{ReentrantLock}) at ./condition.jl:105
 [4] (::var"##19#20")() at ./threadingconstructs.jl:113
```

(The same holds for `@async` as `@spawn`.)

After this change, the assertion works correctly:
```
julia> c = Threads.Condition();

julia> lock(c)

julia> fetch(Threads.@Spawn Base.assert_havelock(c))  # This correctly ERRORs
ERROR: TaskFailedException:
concurrency violation detected
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] concurrency_violation() at ./condition.jl:8
 [3] assert_havelock at ./condition.jl:28 [inlined]
 [4] assert_havelock at ./REPL[22]:1 [inlined]
 [5] assert_havelock(::Base.GenericCondition{ReentrantLock}) at ./condition.jl:73
 [6] (::var"##21#22")() at ./threadingconstructs.jl:113
```

base/lock.jl

@@ -16,6 +16,7 @@ mutable struct ReentrantLock <: AbstractLock
     ReentrantLock() = new(nothing, GenericCondition{Threads.SpinLock}(), 0)
 end
 
+assert_havelock(l::ReentrantLock) = assert_havelock(l, l.locked_by)
 
 """
     islocked(lock) -> Status (Boolean)