mm/gup: do not migrate zero page

On some platforms ZERO_PAGE(0) might end-up in a movable zone.  Do not
migrate zero page in gup during longterm pinning as migration of zero page
is not allowed.

For example, in x86 QEMU with 16G of memory and kernelcore=5G parameter, I
see the following:

Boot#1: zero_pfn  0x48a8d zero_pfn zone: ZONE_DMA32
Boot#2: zero_pfn 0x20168d zero_pfn zone: ZONE_MOVABLE

On x86, empty_zero_page is declared in .bss and depending on the loader
may end up in different physical locations during boots.

Also, move is_zero_pfn() my_zero_pfn() functions under CONFIG_MMU, because
zero_pfn that they are using is declared in memory.c which is compiled
with CONFIG_MMU.

Link: https://lkml.kernel.org/r/20210215161349.246722-9-pasha.tatashin@soleen.com
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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