The LPID allocator init is changed to:
- use mmu_lpid_bits rather than hard-coding;
- use KVM_MAX_NESTED_GUESTS for nested hypervisors;
- not reserve the top LPID on POWER9 and newer CPUs.

The reserved LPID is made a POWER7/8-specific detail.
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
Reviewed-by: NFabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220123120043.3586018-3-npiggin@gmail.com