kernfs: Add noop_fsync to supported kernfs_file_fops

If you edit a kernfs backed file with vi(1), you see an ugly error
message when you write the file because vi tries to fsync(2) the
file after writing, which fails.

We have noop_fsync() for this, use it.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>