The existing logic incorrectly maps this buffer list:

    0: addr 0x10001000, size 0x1000
    1: addr 0x10002000, size 0x1000

To this bogus page list:

    0: 0x10000000
    1: 0x10002000

The shift calculation must also take into account the address of the
first entry masked by the page_mask as well as the last address+size
rounded up to the next page size.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>