MFC: r320062, r320070, r320126
This is a partial merge of only the NFS changes and not the maxbcachebuf
tunable.

The NFS client changes make the code handle different I/O sizes more
correctly. However, with the limit at 64K, they are not actually
necessary.
This MFC is mainly being done so that subsequent MFCs to the NFS
code will merge easily.