xfs: clean up buffer get/read call API

The xfs_buf_get/read API is not consistent in the units it uses, and
does not use appropriate or consistent units/types for the
variables.

Convert the API to use disk addresses and block counts for all
buffer get and read calls. Use consistent naming for all the
functions and their declarations, and convert the internal functions
to use disk addresses and block counts to avoid need to convert them
from one type to another and back again.

Fix all the callers to use disk addresses and block counts. In many
cases, this removes an additional conversion from the function call
as the callers already have a block count.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>