KVM: PPC: Book3S HV: Add a function to filter guest LPCR bits

Guest LPCR depends on hardware type, and future changes will add
restrictions based on errata and guest MMU mode. Move this logic
to a common function and use it for the cases where the guest
wants to update its LPCR (or the LPCR of a nested guest).

This also adds a warning in other places that set or update LPCR
if we try to set something that would have been disallowed by
the filter, as a sanity check.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210412014845.1517916-4-npiggin@gmail.com