This function returns the page offset of the buffer, which can be
calculated based on either its DMA address or its virtual address.  It
used to use the virtual address and we would cast that to unsigned
long, as anything smaller would result in a compiler warning.  Now
that it's using the DMA address we should use unsigned int, matching
the return type.  It is also unnecessary to use __force.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>