selftests/futex: Add FUTEX_LOCK_PI owner-exiting coverage

The futex functional tests cover FUTEX_LOCK_PI timeout semantics
(futex_wait_timeout.c) and robust-list owner death (robust_list.c), but
nothing exercises a non-robust PI owner exiting while holding the lock,
nor the basic ownership / EDEADLK / unlock word semantics of
FUTEX_LOCK_PI.

Add futex_lock_pi_exiting.c with three tests:

 - lock_unlock_basic: an uncontended FUTEX_LOCK_PI puts the owner TID in
   the futex word, a recursive lock by the owner returns EDEADLK, and
   FUTEX_UNLOCK_PI clears the word.

 - owner_dies_with_blocked_waiter: a thread acquires a PI futex and exits
   while holding it; a blocked FUTEX_LOCK_PI waiter must come out cleanly
   (0, EOWNERDEAD or ESRCH) and own the lock when it acquires.

 - stress_owner_exits: repeatedly drive the same exiting-owner path.

The exiting-owner retry path is where commit 210d36d892de ("futex: Clear
stale exiting pointer in futex_lock_pi() retry path") fixed a stale task
pointer that tripped WARN_ON_ONCE() in wait_for_owner_exiting(). That
warning does not change the syscall return value, so this test does not
assert on it directly; the stress loop exists to give a kernel booted
with panic_on_warn=1 (as fuzzers and CI commonly run) a chance to trip
on it.

Signed-off-by: Valery Borovsky <vebohr@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260607183249.246037-1-vebohr@gmail.com

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