net/9p: increase default msize to 128k

Let's raise the default msize value to 128k.

The 'msize' option defines the maximum message size allowed for any
message being transmitted (in both directions) between 9p server and 9p
client during a 9p session.

Currently the default 'msize' is just 8k, which is way too conservative.
Such a small 'msize' value has quite a negative performance impact,
because individual 9p messages have to be split up far too often into
numerous smaller messages to fit into this message size limitation.

A default value of just 8k also has a much higher probablity of hitting
short-read issues like: https://gitlab.com/qemu-project/qemu/-/issues/409

Unfortunately user feedback showed that many 9p users are not aware that
this option even exists, nor the negative impact it might have if it is
too low.

Link: http://lkml.kernel.org/r/61ea0f0faaaaf26dd3c762eabe4420306ced21b9.1630770829.git.linux_oss@crudebyte.com
Link: https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg01003.html
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/client.c

@@ -30,7 +30,7 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/9p.h>
 
-#define DEFAULT_MSIZE 8192
+#define DEFAULT_MSIZE (128 * 1024)
 
 /*
   * Client Option Parsing (code inspired by NFS code)