cmd/upspinfs: make the file permissions an unchangeable 0700
cmd/upspinfs: make the file permissions an unchangeable 0700

The previous behavior was to allow FUSE to change permissions.
The change would last as long as the file was cached by upspinfs
and then fall back to 0700. This change removes that ambiguity
by not letting the mode be changed. However we do not return
an error on mode changes but instead silently ignore them.

Change-Id: I84212103bac21b8786d5f302ee6565804e86cc03
Reviewed-on: https://upspin-review.googlesource.com/8662
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin