mm: relax deferred struct page requirements

There is no need to have ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT, as all
the page initialization code is in common code.

Also, there is no need to depend on MEMORY_HOTPLUG, as initialization
code does not really use hotplug memory functionality.  So, we can
remove this requirement as well.

This patch allows to use deferred struct page initialization on all
platforms with memblock allocator.

Tested on x86, arm64, and sparc.  Also, verified that code compiles on
PPC with CONFIG_MEMORY_HOTPLUG disabled.

Link: http://lkml.kernel.org/r/20171117014601.31606-1-pasha.tatashin@oracle.com


Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>	[s390]
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Steven Sistare <steven.sistare@oracle.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Benjamin Herrenschmidt <benh@ke...