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.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Build-tested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231123071705.397625-1-naveen@kernel.org