exp/inotify: close event channel before file descriptor

Closing the inotify file descriptor can take over a second
when running on Ubuntu Precise in an NFS directory, leading to
the test error in issue 3132.  Closing the event channel first
lets a client that does not care about the error channel move
on.

Fixes issue 3132.

R=golang-dev, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7300045