KVM: PPC: Book3S HV: Remove user-triggerable WARN_ON

Although in general we do not expect valid PTEs to be found in
kvmppc_create_pte when we are inserting a large page mapping, there
is one situation where this can occur.  That is when dirty page
logging is turned off for a memslot while the VM is running.
Because the new memslots are installed before the old memslot is
flushed in kvmppc_core_commit_memory_region_hv(), there is a
window where a hypervisor page fault can try to install a 2MB
(or 1GB) page where there are already small page mappings which
were installed while dirty page logging was enabled and which
have not yet been flushed.

Since we have a situation where valid PTEs can legitimately be
found by kvmppc_unmap_free_pte, and which can be triggered by
userspace, just remove the WARN_ON_ONCE, since it is undesirable
to have userspace able to trigger a kernel warning.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>