powerpc/603: Really copy kernel PGD entries into all PGDIRs

Commit 82ef440f ("powerpc/603: Copy kernel PGD entries into all
PGDIRs and preallocate execmem page tables") was supposed to extend
to powerpc 603 the copy of kernel PGD entries into all PGDIRs
implemented in a previous patch on the 8xx. But 603 is book3s/32 and
uses a duplicate of pgd_alloc() defined in another header.

So really do the copy at the correct place for the 603.

Fixes: 82ef440f ("powerpc/603: Copy kernel PGD entries into all PGDIRs and preallocate execmem page tables")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/752ab7514cae089a2dd7cc0f3d5e35849f76adb9.1755757797.git.christophe.leroy@csgroup.eu