orangefs: migrate to generic_file_read_iter

Remove orangefs_inode_read.  It was used by readpage.  Calling
wait_for_direct_io directly serves the purpose just as well.  There is
now no check of the bufmap size in the readpage path.  There are already
other places the bufmap size is assumed to be greater than PAGE_SIZE.

Important to call truncate_inode_pages now in the write path so a
subsequent read sees the new data.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>