arm64: kvm: Fix IDMAP overlap with HYP VA

Booting 5.4 on LX2160A reveals that KVM is non-functional:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP intersecting with HYP VA, unable to continue
kvm [1]: error initializing Hyp mode: -22

Debugging shows:

kvm [1]: IDMAP page: 81a26000
kvm [1]: HYP VA range: 0:22ffffffff

as RAM is located at:

80000000-fbdfffff : System RAM
2080000000-237fffffff : System RAM

Comparing this with the same kernel on Armada 8040 shows:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP page: 2a26000
kvm [1]: HYP VA range: 4800000000:493fffffff
...
kvm [1]: Hyp mode initialized successfully

which indicates that hyp_va_msb is set, and is always set to the
opposite value of the idmap page to avoid the overlap. This does not
happen with the LX2160A.

Further debugging shows vabits_actual = 39, kva_msb = 38 on LX2160A and
kva_msb = 33 on Armada 8040. Looking at the bit layout of the HYP VA,
there is still one bit available for hyp_va_msb. Set this bit
appropriately. This allows KVM to be functional on the LX2160A, but
without any HYP VA randomisation:

kvm: Limiting the IPA size due to kernel Virtual Address limit
kvm [1]: IPA Size Limit: 43bits
kvm [1]: IDMAP page: 81a24000
kvm [1]: HYP VA range: 4000000000:62ffffffff
...
kvm [1]: Hyp mode initialized successfully

Fixes: ed57cac8 ("arm64: KVM: Introduce EL2 VA randomisation")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
[maz: small additional cleanups, preserved case where the tag
 is legitimately 0 and we can just use the mask, Fixes tag]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/E1ilAiY-0000MA-RG@rmk-PC.armlinux.org.uk