Issue READIRPLUS calls with a sensible dircount maxcount ratio

The NFS spec states:

"The ratio of the directory entry size to the size of the attributes plus the
size of the file handle is usually at least 8 to 1."

Though implementation dependent the spec implies that the previous 1 to 1 ratio
could have a detrimental impact on the performance of the server due to most of
the data per VOP_READDIR call being wasted.

The Linux kernel uses the 8 to 1 ratio.

Signed-off-by: Stefan Blanke <stefan.blanke@framestore.com>
Signed-off-by: Mark Hills <mark.hills@framestore.com>