drop ntb0 MTU to 64000 (#14136)

src/freenas/etc/systemd/network/10-persistent-net.link

@@ -1,5 +1,15 @@
 [Match]
 Driver=ntb_netdev
 
+
 [Link]
 Name=ntb0
+# TODO: at time of writing, ntb on SCALE has pathological
+# behavior related TCP/IP Window size calculation whereby
+# the receiving side clamps the window size to 0 causing
+# recalculation which, ultimately, ends with behavior
+# described in https://en.wikipedia.org/wiki/Silly_window_syndrome
+# The work-around is to drop MTU size a bit which fixes
+# the scenario altogether. When we discover why this is
+# happening, we should remove the MTUBytes line.
+MTUBytes=64000