KVM: arm64: Make MTE_frac masking conditional on MTE capability

If MTE_frac is masked out unconditionally then the guest will always
see ID_AA64PFR1_EL1_MTE_frac as 0. However, a value of 0 when
ID_AA64PFR1_EL1_MTE is 2 indicates that MTE_ASYNC is supported. Hence, for
a host with ID_AA64PFR1_EL1_MTE==2 and ID_AA64PFR1_EL1_MTE_frac==0xf
(MTE_ASYNC unsupported) the guest would see MTE_ASYNC advertised as
supported whilst the host does not support it. Hence, expose the sanitised
value of MTE_frac to the guest and user-space.

As MTE_frac was previously hidden, always 0, and KVM must accept values
from KVM provided by user-space, when ID_AA64PFR1_EL1.MTE is 2 allow
user-space to set ID_AA64PFR1_EL1.MTE_frac to 0. However, ignore it to
avoid incorrectly claiming hardware support for MTE_ASYNC in the guest.

Note that linux does not check the value of ID_AA64PFR1_EL1_MTE_frac and
wrongly assumes that MTE async faults can be generated even on hardware
that does nto support them. This issue is not addressed here.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Link: https://lore.kernel.org/r/20250512114112.359087-3-ben.horgan@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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