NFSD: Add a "default" block size

We'd like to increase the maximum r/wsize that NFSD can support,
but without introducing possible regressions. So let's add a
default setting of 1MB. A subsequent patch will raise the
maximum value but leave the default alone.

No behavior change is expected.

Reviewed-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfsd.h

···
 #include "stats.h"
 
 /*
- * Maximum blocksizes supported by daemon under various circumstances.
+ * Default and maximum payload size (NFS READ or WRITE), in bytes.
+ * The default is historical, and the maximum is an implementation
+ * limit.
  */
-#define NFSSVC_MAXBLKSIZE       RPCSVC_MAXPAYLOAD
+enum {
+	NFSSVC_DEFBLKSIZE       = 1 * 1024 * 1024,
+	NFSSVC_MAXBLKSIZE       = RPCSVC_MAXPAYLOAD,
+};
 
 struct readdir_cd {
 	__be32			err;	/* 0, nfserr, or nfserr_eof */

fs/nfsd/nfssvc.c

···
 	 */
 	target >>= 12;
 
-	ret = NFSSVC_MAXBLKSIZE;
+	ret = NFSSVC_DEFBLKSIZE;
 	while (ret > target && ret >= 8*1024*2)
 		ret /= 2;
 	return ret;