hello-debufs: add foo read/write

Add a 'foo' file under debugfs which can be read from or written to and
stores up to one PAGE_SIZE of data.
hello-debugfs: misc_const: fix incorrect comparision

Fix the comparision on write in both the hello-debugfs driver and the
misc_const driver so that they return a failure when a bad id is
written.  Previously this was backwards (!strncmp).