powerpc/603: Copy kernel PGD entries into all PGDIRs and preallocate execmem page tables

For the same reason as 8xx, copy kernel PGD entries into all
PGDIRs in pgd_alloc() and preallocate execmem page tables before
creating new PGDs so that all PGD entries related to execmem are
copied by pgd_alloc().

This will help reduce the fast-path in TLBmiss handlers.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/1a0d1feee07c4cf955f6a43a704c203e5c90fa53.1724173828.git.christophe.leroy@csgroup.eu