Fix VOP_CLOSE() in userspace.

Currently, for unknown reasons, VOP_CLOSE() is a no-op in userspace.
This causes file descriptor leaks. This is especially problematic with
long ztest runs, since zpool.cache is opened repeatedly and never
closed, resulting in resource exhaustion (EMFILE errors).

This patch fixes the issue by making VOP_CLOSE() do what it is supposed
to do.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #989