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

In the Linux kernel, the following vulnerability has been resolved:

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.