powerpc/603: Fix boot failure with DEBUG_PAGEALLOC and KFENCE

Allthough kernel text is always mapped with BATs, we still have
inittext mapped with pages, so TLB miss handling is required
when CONFIG_DEBUG_PAGEALLOC or CONFIG_KFENCE is set.

The final solution should be to set a BAT that also maps inittext
but that BAT then needs to be cleared at end of init, and it will
require more changes to be able to do it properly.

As DEBUG_PAGEALLOC or KFENCE are debugging, performance is not a big
deal so let's fix it simply for now to enable easy stable application.

Fixes: 035b19a1 ("powerpc/32s: Always map kernel text and rodata with BATs")
Cc: stable@vger.kernel.org # v5.11+
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aea33b4813a26bdb9378b5f273f00bd5d4abe240.1638857364.git.christophe.leroy@csgroup.eu