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