KVM: arm64: Populate fault info for watchpoint

When handling ESR_ELx_EC_WATCHPT_LOW, far_el2 member of struct
kvm_vcpu_fault_info will be copied to far member of struct
kvm_debug_exit_arch and exposed to the userspace. The userspace will
see stale values from older faults if the fault info does not get
populated.

Fixes: 8fb20461

 ("KVM: arm64: Move early handlers to per-EC handlers")
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230530024651.10014-1-akihiko.odaki@daynix.com
Cc: stable@vger.kernel.org