RISC-V: KVM: Fix potential UAF in kvm_riscv_aia_imsic_has_attr()

The KVM_DEV_RISCV_AIA_GRP_APLIC branch of aia_has_attr() was identified
to have a race condition with concurrent KVM_SET_DEVICE_ATTR ioctls,
leading to a use-after-free bug.

Upon analyzing the code, it was discovered that the
KVM_DEV_RISCV_AIA_GRP_IMSIC branch of aia_has_attr() suffers from the same
lack of synchronization. It invokes kvm_riscv_aia_imsic_has_attr() without
holding dev->kvm->lock.

While aia_has_attr() is running, a concurrent aia_set_attr() could call
aia_init() under the dev->kvm->lock. If aia_init() fails, it may trigger
kvm_riscv_vcpu_aia_imsic_cleanup(), which frees imsic_state. Without proper
locking, kvm_riscv_aia_imsic_has_attr() could attempt to access imsic_state
while it is being deallocated.

Although this specific path has not yet been reported by a fuzzer, it
is logically identical to the APLIC issue. Fix this by acquiring the
dev->kvm->lock before calling kvm_riscv_aia_imsic_has_attr(), ensuring
consistency with the locking pattern used for other AIA attribute groups.

Fixes: 5463091a51cf ("RISC-V: KVM: Expose IMSIC registers as attributes of AIA irqchip")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260304080804.2281721-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>