inotify: fix memory leak in Watcher

Watcher.RemoveWatch is failing to clean up the FD to watch path mapping,
leaving them to slowly accumulate over time. This deletes that mapping
while coordinating with the readEvents loop.

Fixes golang/go#16097

Change-Id: I8dcf5212ae54e3adc15347e1ac33000afd862ef7
Reviewed-on: https://go-review.googlesource.com/24289
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
1 file changed