locks: fix a potential use-after-free problem when wakeup a waiter

'16306a61 ("fs/locks: always delete_block after waiting.")' add the
logic to check waiter->fl_blocker without blocked_lock_lock. And it will
trigger a UAF when we try to wakeup some waiter：

Thread 1 has create a write flock a on file, and now thread 2 try to
unlock and delete flock a, thread 3 try to add flock b on the same file.

Thread2                         Thread3
                                flock syscall(create flock b)
	                        ...flock_lock_inode_wait
				    flock_lock_inode(will insert
				    our fl_blocked_member list
				    to flock a's fl_blocked_requests)
				   sleep
flock syscall(unlock)
...flock_lock_inode_wait
    locks_delete_lock_ctx
    ...__locks_wake_up_blocks
        __locks_delete_blocks(
	b->fl_blocker = NULL)
	...
                                   break by a signal
				   locks_delete_block
				    b->fl_blocker == NULL &&
				    list_empty(&b->fl_blocked_requests)
	                            success, return directly
				 locks_free_lock b
	wake_up(&b->fl_waiter)
	trigger UAF

Fix it by remove this logic, and this patch may also fix CVE-2019-19769.

Cc: stable@vger.kernel.org
Fixes: 16306a61

 ("fs/locks: always delete_block after waiting.")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>