This reverts commit 6f18dc89.

Just as one example, it appears this code could do the wrong thing in
the case of a two-byte NFS READ that crosses a page boundary.

Chuck says: "In that case, nfsd would pass down an xdr_buf that has one
byte in a page, one byte in another page, and a two-byte XDR pad. The
logic introduced by this optimization would be fooled, and neither the
second byte nor the XDR pad would be written to the client."

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>