selftests/futex: Create test for robust list

Create a test for the robust list mechanism. Test the following uAPI
operations:

- Creating a robust mutex where the lock waiter is wake by the kernel
  when the lock owner died
- Setting a robust list to the current task
- Getting a robust list from the current task
- Getting a robust list from another task
- Using the list_op_pending field from robust_list_head struct to test
  robustness when the lock owner dies before completing the locking
- Setting a invalid size for syscall argument `len`
- Adding multiple elements to a robust list wait waiting for each of
  them
- Creating a circular list and checking that the kernel does not get
  stuck in an infinity loop

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251110224130.3044761-1-andrealmeid@igalia.com

Author: andrealmeid

tools/testing/selftests/futex/functional/.gitignore

@@ -12,3 +12,4 @@ futex_wait_uninitialized_heap
 futex_wait_wouldblock
 futex_waitv
 futex_numa
+robust_list

tools/testing/selftests/futex/functional/Makefile

@@ -22,7 +22,8 @@ TEST_GEN_PROGS := \
 	futex_priv_hash \
 	futex_numa_mpol \
 	futex_waitv \
-	futex_numa
+	futex_numa \
+	robust_list
 
 TEST_PROGS := run.sh