libxfs: avoid dependency on Linux XATTR_SIZE_MAX

libxfs: avoid dependency on Linux XATTR_SIZE_MAX

Currently, we depends on Linux XATTR value for on disk
definitions. Which causes trouble on other platforms and
maybe also if this value was to change.

Fix it by creating a custom definition independent from
those in Linux (although with the same values), so it is OK
with the be16 fields used for holding these attributes.

Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>