error = dmu_sync(zio, lr->lr_common.lrc_txg,
                            zfs_get_done, zgd);
                       ASSERT(error || lr->lr_length <= zp->z_blksz);
It's possible, although extremely rare, that the zfs_get_done() callback is
executed before dmu_sync() returns.
In that case the znode's range lock is dropped and the znode is unreferenced.
Thus, the assertion can access some invalid or wrong data via the zp pointer.
size variable caches the correct value of z_blksz and can be safely used here.