arm64: mops: Do not dereference src reg for a set operation

The source register is not used for SET* and reading it can result in
a UBSAN out-of-bounds array access error, specifically when the MOPS
exception is taken from a SET* sequence with XZR (reg 31) as the
source. Architecturally this is the only case where a src/dst/size
field in the ESR can be reported as 31.

Prior to 2de451a329cf662b the code in do_el0_mops() was benign as the
use of pt_regs_read_reg() prevented the out-of-bounds access.

Fixes: 2de451a329cf ("KVM: arm64: Add handler for MOPS exceptions")
Cc: <[email protected]> # 6.12.x
Cc: Kristina Martsenko <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Reviewed-by: Marc Zyngier <[email protected]>
Signed-off-by: Keir Fraser <[email protected]>
Reviewed-by: Kristina Martšenko <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

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