powerpc/lib: Validate size for vector operations

In the Linux kernel, the following vulnerability has been resolved: powerpc/lib: Validate size for vector operations Some of the fp/vmx code in sstep.c assume a certain maximum size for the instructions being emulated. The size of those operations however is determined separately in analyse_instr(). Add a check to validate the assumption on the maximum size of the operations, so as to prevent any unintended kernel stack corruption.