powerpc: Mark writes registering ipi to host cpu through kvm and polling

Mark writes to hypervisor ipi state so that KCSAN recognises these
asynchronous issue of kvmppc_{set,clear}_host_ipi to be intended, with
atomic writes. Mark asynchronous polls to this variable in
kvm_ppc_read_one_intr().

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230510033117.1395895-9-rmclure@linux.ibm.com