xfs: factor projid hint checking out of xfs_ioctl_setattr

The project ID change checking is one of the few remaining open
coded checks in xfs_ioctl_setattr(). Factor it into a helper
function so that the setattr code mostly becomes a flow of check
and action helpers, making it easier to read and follow.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>