KVM: arm64: Introduce helper to calculate fault IPA offset

KVM: arm64: Introduce helper to calculate fault IPA offset

This 12-bit FAR fault IPA offset mask is hard-coded as 'GENMASK(11, 0)'
in several places to reconstruct the full fault IPA.

Introduce FAR_TO_FIPA_OFFSET() to calculate this value in a shared
header and replace all open-coded instances to improve readability.

No functional change intended.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20251211104710.151771-5-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>