netfs: Fix double put of request

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

netfs: Fix double put of request

If a netfs request finishes during the pause loop, it will have the ref
that belongs to the IN_PROGRESS flag removed at that point - however, if it
then goes to the final wait loop, that will *also* put the ref because it
sees that the IN_PROGRESS flag is clear and incorrectly assumes that this
happened when it called the collector.

In fact, since IN_PROGRESS is clear, we shouldn't call the collector again
since it's done all the cleanup, such as calling ->ki_complete().

Fix this by making netfs_collect_in_app() just return, indicating that
we're done if IN_PROGRESS is removed.