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: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>