CVE-2026-89775: KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation

Computing the effects of a TLB invalidation involves looking at the size of the mapping cached by the TLB. For S1 mappings such as VNCR, this is deducted from the combination of the base granule size and the mapping level.

However, this implies that the S1 MMU is on. When the MMU is off, we indicate this with the level being set to a "creative" value of -127 (S1MMUDISABLED).

This ends-up being misinterpreted by pgshiftleveltottl() as it doesn't handle negative levels at all (the level is immediately cast to a u8 and only the bottom two bits considered), leading to an invalidation size of 0. Not helpful.

Tidy-up pgshiftleveltottl() to handle these negative levels, and ttltosize() to always return SZ1G when no valid TTL is present. This allows the removal of open-coded checks for similar situations.

Note that the check for a negative value not explicitely checking for S1MMUDISABLED is deliberate, so that actual negative levels introduced with LVA2 and D128 can take the same path if we ever support them.