xfs: cleanup struct xfs_attr_list_context

Replace the alist char pointer with a void buffer given that different
callers use it in different ways.  Use the chance to remove the typedef
and reduce the indentation of the struct definition so that it doesn't
overflow 80 char lines all over.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>