Add a benchmark for attaches.
Add a benchmark for attaches.

This uses the new nullfs and runs an attach benchmark.

Attaches seem to take 122 microseconds. That seems long but a quick look
at pprof doesn't show any smoking guns.

Signed-off-by: Ronald G. Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

lionkov/ninep