mav@ has found that NFS servers exporting ZFS file systems
can perform better when using a 128K read/write data size.
This patch changes NFS_MAXDATA from 64K to 128K so that
clients can use 128K for NFS mounts to allow this.
The patch also renames NFS_MAXDATA to NFS_SRVMAXIO so
that it is clear that it applies to the NFS server side
only. It also avoids a name conflict with the NFS_MAXDATA
defined in rpcsvc/nfs_prot.h, that is used for userland RPC.

Tested by: mav
Reviewed by: mav
MFC after: 2 weeks