powerpc/lib/code-patching: Don't use struct 'ppc_inst' for runnable code in tests.

'struct ppc_inst' is meant to represent an instruction internally, it
is not meant to dereference code in memory.

For testing code patching, use patch_instruction() to properly
write into memory the code to be tested.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d8425fb42a4adebc35b7509f121817eeb02fac31.1621516826.git.christophe.leroy@csgroup.eu