CVE-2024-41023: sched/deadline: Fix task_struct reference leak

In the Linux kernel, the following vulnerability has been resolved:

sched/deadline: Fix taskstruct reference leak

During the execution of the following stress test with linux-rt:

stress-ng --cyclic 30 --timeout 30 --minimize --quiet

kmemleak frequently reported a memory leak concerning the taskstruct:

unreferenced object 0xffff8881305b8000 (size 16136): comm "stress-ng", pid 614, jiffies 4294883961 (age 286.412s) object hex dump (first 32 bytes): 02 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 .@.............. 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ debug hex dump (first 16 bytes): 53 09 00 00 00 00 00 00 00 00 00 00 00 00 00 00 S............... backtrace: [<00000000046b6790>] duptaskstruct+0x30/0x540 [<00000000c5ca0f0b>] copyprocess+0x3d9/0x50e0 [<00000000ced59777>] kernelclone+0xb0/0x770 [<00000000a50befdc>] dosysclone+0xb6/0xf0 [<000000001dbf2008>] dosyscall64+0x5d/0xf0 [<00000000552900ff>] entrySYSCALL64afterhwframe+0x6e/0x76

The issue occurs in startdltimer(), which increments the taskstruct reference count and sets a timer. The timer callback, dltasktimer, is supposed to decrement the reference count upon expiration. However, if enqueuetaskdl() is called before the timer expires and cancels it, the reference count is not decremented, leading to the leak.

This patch fixes the reference leak by ensuring the taskstruct reference count is properly decremented when the timer is canceled.