LoongArch: Don't crash in stack_top() for tasks without vDSO

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

LoongArch: Don't crash in stack_top() for tasks without vDSO

Not all tasks have a vDSO mapped, for example kthreads never do. If such

a task ever ends up calling stack_top(), it will derefence the NULL vdso

pointer and crash.

This can for example happen when using kunit:

[<9000000000203874>] stack_top+0x58/0xa8

[<90000000002956cc>] arch_pick_mmap_layout+0x164/0x220

[<90000000003c284c>] kunit_vm_mmap_init+0x108/0x12c

[<90000000003c1fbc>] __kunit_add_resource+0x38/0x8c

[<90000000003c2704>] kunit_vm_mmap+0x88/0xc8

[<9000000000410b14>] usercopy_test_init+0xbc/0x25c

[<90000000003c1db4>] kunit_try_run_case+0x5c/0x184

[<90000000003c3d54>] kunit_generic_run_threadfn_adapter+0x24/0x48

[<900000000022e4bc>] kthread+0xc8/0xd4

[<9000000000200ce8>] ret_from_kernel_thread+0xc/0xa4