mm: highmem: export kmap_to_page for modules

Some virtio device drivers (9p) need to translate high virtual addresses
to physical addresses, which are inserted into the virtqueue for
processing by userspace.

This patch exports the kmap_to_page symbol, so that the affected drivers
can be compiled as modules.

Cc: stable@kernel.org
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

mm/highmem.c

@@ -105,6 +105,7 @@ struct page *kmap_to_page(void *vaddr)
 
 	return virt_to_page(addr);
 }
+EXPORT_SYMBOL(kmap_to_page);
 
 static void flush_all_zero_pkmaps(void)
 {