arm64: traps: correctly handle MRS/MSR with XZR

Currently we hand-roll XZR-safe register handling in
user_cache_maint_handler(), though we forget to do the same in
ctr_read_handler(), and may erroneously write back to the user SP rather
than XZR.

Use the new helpers to handle these cases correctly and consistently.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 116c81f4 ("arm64: Work around systems with mismatched cache line sizes")
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>