xfs: remove struct xfs_attr_item.xattri_flags

Nobody uses this field, so get rid of it and the unused flag definition.
Rearrange the structure layout to reduce its size from 104 to 96 bytes.
This gets us from 39 to 42 objects per page.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>