So that e.g. File.Read could be canceled.

This is already so in fs/ package. However WCFS in Wendelin.core still
uses older nodefs package, and for now it was faster to teach old
nodefs.File about cancel propagation.

References:

wendelin.core@b17aeb8c
wendelin.core@f05271b1
wendelin.core@5ba816da
go123@7ad867a3
go123@0bdac628
go123@d2dc6c09

Was proposed to upstream in FYI mode:

https://github.com/hanwen/go-fuse/pull/343
https://review.gerrithub.io/c/hanwen/go-fuse/+/488787

Change-Id: Ia9f49f04d87ad3d42f1e5b14c62fe9b425f404a9