selftests: add file SLAB_TYPESAFE_BY_RCU recycling stressor

Add a simple file stressor that lives directly in-tree. This will create
a bunch of processes that each open 500 file descriptors and then use
close_range() to close them all.

Concurrently, other processes read /proc/<pid>/fd/ which rougly does

    f = fget_task_next(p, &fd);
    if (!f)
           break;
    data.mode = f->f_mode;
    fput(f);

Which means that it'll try to get a reference to a file in another
task's file descriptor table.

Under heavy file load it is increasingly likely that the other task will
manage to close @file and @file will be recycled due to
SLAB_TYPEAFE_BY_RCU concurrently. This will trigger various warnings in
the file reference counting code.

Link: https://lore.kernel.org/r/20241021-vergab-streuen-924df15dceb9@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/filesystems/.gitignore

@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
 dnotify_test
 devpts_pts
+file_stressor