[patch] fix add_bounce_page() for BUS_DMA_KEEP_PG_OFFSET

The add_bounce_page() can be called from _bus_dmamap_load_phys().
Client virtual address is zero (not valid) in that case. So, client
physical address must be used to get an offset when
BUS_DMA_KEEP_PG_OFFSET flag is set. Note that client physical address
is always valid.

Svatopluk Kraus