Cleanup: Delete dead code from send_merge_thread()

range is always deferenced before it reaches this check, such that the
kmem_zalloc() call is never executed.

A previously version of this had erronously also pruned the
`range->eos_marker = B_TRUE` line, but it must be set whenever we
encounter an error or are cancelled early.

Coverity incorrectly complained about a potential NULL pointer
dereference because of this.

Reported-by: Coverity (CID 1524550)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14210

module/zfs/dmu_send.c

@@ -1586,8 +1586,6 @@ send_merge_thread(void *arg)
 		}
 		range_free(front_ranges[i]);
 	}
-	if (range == NULL)
-		range = kmem_zalloc(sizeof (*range), KM_SLEEP);
 	range->eos_marker = B_TRUE;
 	bqueue_enqueue_flush(&smt_arg->q, range, 1);
 	spl_fstrans_unmark(cookie);