OpenZFS: xattr=sa is ignored on FreeBSD

FreeBSD does not read and write SA style xattrs, it simply treats xattr=sa as xattr=on and uses the xattr dir. We set xattr=sa by default in TrueNAS. The xattrs written on SCALE will therefore not be accessible on FreeBSD. Linux does fall back to the xattr dir when looking up xattrs, so xattrs written on CORE are still accessible on SCALE, they're just not stored in the expected way. The performance benefits of xattr=sa will therefore not be realized.