powerpc/8xx: Inconditionally use task PGDIR in DTLB misses

At the time being, DATA TLB miss handlers use task PGDIR for user
addresses and swapper_pg_dir for kernel addresses.

Now that kernel part of swapper_pg_dir is copied into task PGDIR
at PGD allocation, it is possible to avoid the above logic and
always use task PGDIR.

But new kernel PGD entries can still be created after init, in
which case those PGD entries may miss in task PGDIR. This can be
handled in DATA TLB error handler.

However, it needs to be done in real mode because the missing
entry might be related to the stack.

So implement copy of missing PGD entry in the prolog of DATA TLB
ERROR handler just after the fixup of DAR.

Note that this is feasible because 8xx doesn't implement vmap or
ioremap with 8Mbytes pages but only 512kbytes pages which are at
PTE level.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/7a76a923d2a111f1d843d8b20b4df0c65d2f4a7b.1724173828.git.christophe.leroy@csgroup.eu