Disable get_numeric_property for xattr on FreeBSD

FreeBSD doesn't have a mount flag for determining the
disposition of xattr. Disable so that it is fetched
by the default route so that 'zfs get xattr' returns
the correct value.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9862

lib/libzfs/libzfs_dataset.c

@@ -2344,7 +2344,9 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 	case ZFS_PROP_EXEC:
 	case ZFS_PROP_READONLY:
 	case ZFS_PROP_SETUID:
+#ifndef __FreeBSD__
 	case ZFS_PROP_XATTR:
+#endif
 	case ZFS_PROP_NBMAND:
 		*val = getprop_uint64(zhp, prop, source);