KVM: arm64: nv: Fix handling of XN[0] when !FEAT_XNX

KVM: arm64: nv: Fix handling of XN[0] when !FEAT_XNX

XN has already been extracted from its bitfield position so using
FIELD_PREP() on the mask that clears XN[0] is completely broken, having
the effect of unconditionally granting execute permissions...

Fix the obvious mistake by manipulating the right bit.

Cc: stable@vger.kernel.org
Fixes: d93febe2ed2e ("KVM: arm64: nv: Forward FEAT_XNX permissions to the shadow stage-2")
Reviewed-by: Wei-Lin Chang <weilin.chang@arm.com>
Signed-off-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/20260602165901.52800-2-oupton@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>