xfs: clean up bufsize alignment in xfs_ioc_attr_list

Use the round_down macro, and use the size of the uint32 type we
use in the callback that fills the buffer to make the code a little
more clear - the size of it is always the same as int for platforms
that Linux runs on.

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