inotify: fix race in Close() (#465)

inotify: fix race in Close() (#465)

Would sometimes fail with:

	panic: close of closed channel

	goroutine 204 [running]:
	github.com/fsnotify/fsnotify.(*Watcher).Close(0xc0003e6410)
        	/home/martin/code/Golib/fsnotify/inotify.go:82 +0x66
	created by github.com/fsnotify/fsnotify.TestCloseRace
        	/home/martin/code/Golib/fsnotify/integration_test.go:1256 +0x1a5
	exit status 2

Because isClosed() might return "false" for two goroutines, and then
they will both try to close it, resulting in the panic.

Fixes #367