xfs: only set extent size hint when asked

Currently the extent size hint is set unconditionally in
xfs_ioctl_setattr() when the FSX_EXTSIZE flag is set. Hence we can
set hints when the inode flags indicating the hint should be used
are not set.  Hence only set the extent size hint from userspace
when the inode has the XFS_DIFLAG_EXTSIZE flag set to indicate that
we should have an extent size hint set on the inode.

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