PCI/P2PDMA: Rename upstream_bridge_distance() and rework doc

The function upstream_bridge_distance() has evolved such that its name is
no longer entirely reflective of what the function does.  It not only
calculates the distance between two peers but also calculates how the DMA
addresses for those two peers should be mapped.

Rename it to calc_map_type_and_dist() and rework the documentation to
better describe the two pieces of information the function returns.

[bhelgaas: tweak comment wording]
Link: https://lore.kernel.org/r/20210610160609.28447-2-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>