KVM: x86: Handle SRCU initialization failure during page track init

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

KVM: x86: Handle SRCU initialization failure during page track init

Check the return of init_srcu_struct(), which can fail due to OOM, when

initializing the page track mechanism. Lack of checking leads to a NULL

pointer deref found by a modified syzkaller.

[Move the call towards the beginning of kvm_arch_init_vm. - Paolo]