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This gives us a little bit of extra typesafety as we know that nobody
called virt_to_page() instead of virt_to_head_page().

[ vbabka@suse.cz: Use folio as intermediate step when filtering out
  large kmalloc pages ]
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Roman Gushchin <guro@fb.com>