nfscl: Partially revert commit 867c27c23a5c

Commit 867c27c23a5c enabled the n_directio_opens code
in open/close, which sets/clears NNONCACHE, for
IO_APPEND. This code should not be enabled unless
newnfs_directio_enable is non-zero.

This patch reverts that part of commit 867c27c23a5c.

A future patch that fixes the case where the
file that is being written IO_APPEND is mmap()'d.

MFC after: 3 months