riscv: kvm: Fix out-of-bounds array access

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

riscv: kvm: Fix out-of-bounds array access

In kvm_riscv_vcpu_sbi_init() the entry->ext_idx can contain an

out-of-bound index. This is used as a special marker for the base

extensions, that cannot be disabled. However, when traversing the

extensions, that special marker is not checked prior indexing the

array.

Add an out-of-bounds check to the function.