fuse: Initialize beyond-EOF page contents before setting uptodate

In the Linux kernel, the following vulnerability has been resolved:

fuse: Initialize beyond-EOF page contents before setting uptodate

fuse_notify_store(), unlike fuse_do_readpage(), does not enable page

zeroing (because it can be used to change partial page contents).

So fuse_notify_store() must be more careful to fully initialize page

contents (including parts of the page that are beyond end-of-file)

before marking the page uptodate.

The current code can leave beyond-EOF page contents uninitialized, which

makes these uninitialized page contents visible to userspace via mmap().

This is an information leak, but only affects systems which do not

enable init-on-alloc (via CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y or the

corresponding kernel command line parameter).