clean up physical merging helpers V2

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens,

this series moves various helpers related to merging based on physical
addresses from the public headers into block/, cleans up the code a bit
and removes not nessecary includes from the block headers.

Change since V1:
 - dropped the Xen related changed which are moved into a new series
   to be sent after this one