KVM: arm64: Simplify handling of negative FGT bits

KVM: arm64: Simplify handling of negative FGT bits

check_fgt_bit() and triage_sysreg_trap() implement the same thing
twice for no good reason. We have to lookup the FGT register twice,
as we don't communicate it. Similarly, we extract the register value
at the wrong spot.

Reorganise the code in a more logical way so that things are done
at the correct location, removing a lot of duplication.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>