Coverity spotted this.

Granted, we *just* checked xfs_inod_dquot() in the caller (by
calling xfs_quota_need_throttle). However, this is the only place we
don't check the return value but the check is cheap and future-proof
so add it.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>