selftests/filesystems: add a helper that holds a readdir in a page fault

A getdents64() whose buffer is a page registered with userfaultfd sits
in handle_userfault() with whatever iterate_dir() took before it copied
the entries. Add readdir_hold.h for tests that want to know what that
blocks: it opens the userfaultfd before the test enters a user
namespace, the fault happens in the kernel and needs CAP_SYS_PTRACE in
the initial one, starts the readdir in a thread, waits until that
thread is stuck, queues a create behind it and waits for it to settle,
then probes a lookup with a watchdog and says whether it came back.
The page is released afterwards so that everything drains on a kernel
that still holds the lock across the copy.

The two users follow.

Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-20-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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