SUNRPC: Bump the maximum payload size for the server

Increase the maximum server-side RPC payload to 4MB. The default
remains at 1MB.

An API to adjust the operational maximum was added in 2006 by commit
596bbe53 ("[PATCH] knfsd: Allow max size of NFSd payload to be
configured"). To adjust the operational maximum using this API, shut
down the NFS server. Then echo a new value into:

  /proc/fs/nfsd/max_block_size

And restart the NFS server.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>