net: use writev when writing frames to tap

This allows us to avoid an extra user-space copy of the packet we are
transmitting to the tap. We use directly the buffers described by the
`DescriptorChain` by means of `IoVecBuffer`s.

Signed-off-by: Babis Chalios <[email protected]>

Author: bchalios

resources/seccomp/aarch64-unknown-linux-musl.json

@@ -24,6 +24,10 @@
             {
                 "syscall": "write"
             },
+            {
+                "syscall": "writev",
+                "comment": "Used by the VirtIO net device to write to tap"
+            },
             {
                 "syscall": "fsync"
             },

resources/seccomp/x86_64-unknown-linux-musl.json

@@ -24,6 +24,10 @@
             {
                 "syscall": "write"
             },
+            {
+                "syscall": "writev",
+                "comment": "Used by the VirtIO net device to write to tap"
+            },
             {
                 "syscall": "fsync"
             },