xfs: factor extsize hint checking out of xfs_ioctl_setattr

The extent size hint change checking is fairly complex, so isolate
that into it's own function. This simplifies the logic flow of the
setattr code, making it easier to read.

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