x86: Move init_gbpages() to setup_arch()

The init_gbpages() function is conditionally called from
init_memory_mapping() function. There are two call-sites where
this 'after_bootmem' condition can be true: setup_arch() and
mem_init() via pci_iommu_alloc().

Therefore, it's safe to move the call to init_gbpages() to
setup_arch() as it's always called before mem_init().

This removes an after_bootmem use - paving the way to remove
all uses of that state variable.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <Pine.LNX.4.64.0906221731210.19474@melkki.cs.Helsinki.FI>
Signed-off-by: Ingo Molnar <mingo@elte.hu>