RISC-V: KVM: Fix null pointer dereference in kvm_riscv_vcpu_aia_rmw_topei()

kvm_riscv_vcpu_aia_rmw_topei() assumes that the per-vCPU IMSIC state has
been initialized once AIA is reported as available and initialized at
the VM level. This assumption does not always hold.

Under fuzzed ioctl sequences, a guest may access the IMSIC TOPEI CSR
before the vCPU IMSIC state is set up. In this case,
vcpu->arch.aia_context.imsic_state is still NULL, and the TOPEI RMW path
dereferences it unconditionally, leading to a host kernel crash.

The crash manifests as:
Unable to handle kernel paging request at virtual address
dfffffff0000000e
...
kvm_riscv_vcpu_aia_imsic_rmw arch/riscv/kvm/aia_imsic.c:909
kvm_riscv_vcpu_aia_rmw_topei arch/riscv/kvm/aia.c:231
csr_insn arch/riscv/kvm/vcpu_insn.c:208
system_opcode_insn arch/riscv/kvm/vcpu_insn.c:281
kvm_riscv_vcpu_virtual_insn arch/riscv/kvm/vcpu_insn.c:355
kvm_riscv_vcpu_exit arch/riscv/kvm/vcpu_exit.c:230
kvm_arch_vcpu_ioctl_run arch/riscv/kvm/vcpu.c:1008
...

Fix this by explicitly checking whether the vCPU IMSIC state has been
initialized before handling TOPEI CSR accesses. If not, forward the CSR
emulation to user space.

Fixes: db8b7e97d6137 ("RISC-V: KVM: Add in-kernel virtualization of AIA IMSIC")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260226085119.643295-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>