nfs: accept CONFIG_NFS_READ_SIZE from config file

To take advantage of defragmented packets, the config file
can define CONFIG_NFS_READ_SIZE to override the 1kB default.
No support is there for an environment variable by now.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>

net/nfs.h

@@ -38,8 +38,14 @@
 
 /* Block size used for NFS read accesses.  A RPC reply packet (including  all
  * headers) must fit within a single Ethernet frame to avoid fragmentation.
- * Chosen to be a power of two, as most NFS servers are optimized for this.  */
-#define NFS_READ_SIZE   1024
+ * However, if CONFIG_IP_DEFRAG is set, the config file may want to use a
+ * bigger value. In any case, most NFS servers are optimized for a power of 2.
+ */
+#ifdef CONFIG_NFS_READ_SIZE
+#define NFS_READ_SIZE CONFIG_NFS_READ_SIZE
+#else
+#define NFS_READ_SIZE 1024 /* biggest power of two that fits Ether frame */
+#endif
 
 #define NFS_MAXLINKDEPTH 16