powerpc/mm: fix erroneous duplicate slb_addr_limit init

Commit 67fda38f ("powerpc/mm: Move slb_addr_linit to
early_init_mmu") moved slb_addr_limit init out of setup_arch().

Commit 70110186 ("powerpc/mm: Reduce memory usage for mm_context_t
for radix") brought it back into setup_arch() by error.

This patch reverts that erroneous regress.

Fixes: 70110186 ("powerpc/mm: Reduce memory usage for mm_context_t for radix")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>