xfs: only assert new size for datafork during truncate extents

xfs: only assert new size for datafork during truncate extents

The assertion functions properly because we currently only truncate the
attr to a zero size. Any other new size of the attr is not preempted.
Make this assertion is specific to the datafork, preparing for
subsequent patches to truncate the attribute to a non-zero size.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Long Li <leo.lilong@huawei.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>