powerpc: Handle error in mark_rodata_ro() and mark_initmem_nx()

mark_rodata_ro() and mark_initmem_nx() use functions that can
fail like set_memory_nx() and set_memory_ro(), leading to a not
protected kernel.

In case of failure, panic.

Link: https://github.com/KSPP/linux/issues/7


Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b16329611deb89e1af505d43f0e2a91310584d26.1710587887.git.christophe.leroy@csgroup.eu