KVM: s390: Fix IRQ injection with SIGP Stop and Store Status

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

KVM: s390: Fix IRQ injection with SIGP Stop and Store Status

When __inject_sigp_stop() is called for a Stop and Store Status
operation, if the vCPU is running, the interrupt is marked as pending
and the status is stored by the thread performing the KVM_RUN IOCTL.

If the vCPU is already stopped, the status is stored immediately.

Storing the status means writing into userspace, which might fault, and
__inject_sigp_stop() is called from do_inject_vcpu() which in turn is
always called holding a spinlock, which is obviously an issue.

Fix this by returning -EWOULDBLOCK from __inject_sigp_stop(), and
adding a bool flag to indicate whether a store status is needed. The
callers of do_inject_vcpu() are modified to pass the pointer to the
bool flag; whenever a Store Status operation is needed, the callers can
now perform it outside the spinlock.

Opportunistically refactor kvm_s390_set_irq_state() to use
scoped_guard() and __free().

[ Added Fixes tag while picking -- Claudio ]