Handle EOPNOTSUPP as an indicator that Linux attrs are not supported.

Add EOPNOTSUPP to the set of errno values in _add_linux_attr() that
indicate Linux attrs are not supported by the target filesystem.  The
problem was observed on ZFS-on-Linux, 0.6.2.

Thanks to Will Rouesnel <w.rouesnel@gmail.com> for the report and an
initial patch.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>