mm/vmalloc.c: support HIGHMEM pages in vmap_pages_range_noflush()

If the pages being mapped are in HIGHMEM, page_address() returns NULL. 
This probably wasn't noticed before because there aren't currently any
architectures with HAVE_ARCH_HUGE_VMALLOC and HIGHMEM, but it's simpler to
call page_to_phys() and futureproofs us against such configurations
existing.

Link: https://lkml.kernel.org/r/Yv6qHc6e+m7TMWhi@casper.infradead.org
Fixes: 121e6f3258 ("mm/vmalloc: hugepage vmalloc mappings")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -590,7 +590,7 @@ int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
 		int err;
 
 		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
-					__pa(page_address(pages[i])), prot,
+					page_to_phys(pages[i]), prot,
 					page_shift);
 		if (err)
 			return err;