drm/amdgpu: flush pending RCU callbacks on module unload

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

drm/amdgpu: flush pending RCU callbacks on module unload

Call rcu_barrier() in module exit to wait for outstanding call_rcu() callbacks
before freeing module text, preventing late callback execution in freed memory.

BUG: unable to handle page fault for address: ffffffffc1d59c40
PGD 6a12067 P4D 6a12067 PUD 6a14067 PMD 13698b067 PTE 0
Oops: 0010 [#1] SMP NOPTI
RIP: 0010:0xffffffffc1d59c40
Code: Unable to access opcode bytes at RIP 0xffffffffc1d59c16.
RSP: 0018:ffffc900198c0f28 EFLAGS: 00010286
RAX: ffffffffc1d59c40 RBX: ffff897c7d6b61c0 RCX: ffff88826aff4590
RDX: ffff8884d8b35490 RSI: ffffc900198c0f30 RDI: ffff88812af67290
RBP: 000000000000000a (DONE segment entries) R08: 0000000000000000 R09: 0000000000000100
R10: 0000000000000000 R11: ffffffff82a06100 R12: ffff88811a4e3700
R13: 0000000000000000 R14: ffff897c7d6b6270 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff897c7d680000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffc1d59c16 CR3: 00000104a980a001 CR4: 0000000002770ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<IRQ>
? rcu_do_batch+0x163/0x450
? rcu_core+0x177/0x1c0
? __do_softirq+0xc1/0x280
? asm_call_irq_on_stack+0xf/0x20
</IRQ>
? do_softirq_own_stack+0x37/0x50
? irq_exit_rcu+0xc4/0x100
? sysvec_apic_timer_interrupt+0x36/0x80
? asm_sysvec_apic_timer_interrupt+0x12/0x20
? cpuidle_enter_state+0xd4/0x360
? cpuidle_enter+0x29/0x40
? cpuidle_idle_call+0x108/0x1a0
? do_idle+0x77/0xf0
? cpu_startup_entry+0x19/0x20
? secondary_startup_64_no_verify+0xbf/0xcb

(cherry picked from commit feaa5039f6c12acc9aa934c2d45dcd251a12c69f)