xdp: document xdp_do_flush() before napi_complete_done()

Document in the XDP_REDIRECT manual section that drivers must call
xdp_do_flush() before napi_complete_done(). The two reasons behind
this can be found following the links below.

Signed-off-by: Magnus Karlsson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/all/[e-mail]/
Acked-by: Toke Høiland-Jørgensen <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/filter.c

···
  *    bpf_redirect_info to actually enqueue the frame into a map type-specific
  *    bulk queue structure.
  *
- * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
- *    which will flush all the different bulk queues, thus completing the
- *    redirect.
+ * 3. Before exiting its NAPI poll loop, the driver will call
+ *    xdp_do_flush(), which will flush all the different bulk queues,
+ *    thus completing the redirect. Note that xdp_do_flush() must be
+ *    called before napi_complete_done() in the driver, as the
+ *    XDP_REDIRECT logic relies on being inside a single NAPI instance
+ *    through to the xdp_do_flush() call for RCU protection of all
+ *    in-kernel data structures.
  */
 /*
  * Pointers to the map entries will be kept around for this whole sequence of