db: separate out straight buffer IO from map based IO.

db: separate out straight buffer IO from map based IO.

Libxfs has two different interfaces for getting and reading buffers.
The first is a block/length interface for reading contiguous
regions, and the second is based on extent based xfs_buf_map arrays
for discontiguous regions. The xfs-db code is solely based on a
basic block array interface regardless of the type of region being
read, and so doesn't match to either libxfs interface.

As a first step to converting xfs_db to the libxfs interfaces, add a
simple block/length buffer API and implement it using pread/pwrite.
Then remove the single region conditionals from the basic block array
based interfaces, and convert all the contiguous block read cases to
use the new API.

This new API is temporary - it will be replaced by the equivalent
libxfs interface calls once all the infrastructure preparation for
the changeover has been completed.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>