KVM: arm64: Account for RESx bits in __compute_fgt()

When computing Fine Grained Traps, it is preferable to account for
the reserved bits. The HW will most probably ignore them, unless the
bits have been repurposed to do something else.

Use caution, and fold our view of the reserved bits in,

Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Fixes: c259d763e6b09 ("KVM: arm64: Account for RES1 bits in DECLARE_FEAT_MAP() and co")
Link: https://sashiko.dev/#/patchset/20260319154937.3619520-1-sascha.bischoff%40arm.com
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260401103611.357092-6-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>