net/9p: increase tcp max msize to 1MB

Historically TCP has been limited to 64K buffers, but increasing
msize provides huge performance benefits especially as latency
increase so allow for bigger buffers.

Ideally further improvements could change the allocation from the
current contiguous chunk in slab (kmem_cache) to some scatter-gather
compatible API...

Note this only increases the max possible setting, not the default
value.

Link: http://lkml.kernel.org/r/YTQB5jCbvhmCWzNd@codewreck.org
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_fd.c

@@ -34,7 +34,7 @@
 #include <linux/syscalls.h> /* killme */
 
 #define P9_PORT 564
-#define MAX_SOCK_BUF (64*1024)
+#define MAX_SOCK_BUF (1024*1024)
 #define MAXPOLLWADDR	2
 
 static struct p9_trans_module p9_tcp_trans;