CVE-2026-64069: netfs: Fix cancellation of a DIO and single read subrequests

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

netfs: Fix cancellation of a DIO and single read subrequests

When the preparation of a new subrequest for a read fails, if the subrequest has already been added to the stream->subrequests list, it can't simply be put and abandoned as the collector may see it. Also, if it hasn't been queued yet, it has two outstanding refs that both need to be put. Both DIO read and single-read dispatch fail at this; further, both differ in the order they do things to the way buffered read works.

Fix cancellation of both DIO-read and single-read subrequests that failed preparation by the following steps:

(1) Harmonise all three reads (buffered, dio, single) to queue the subreq before prepping it.

(2) Make all three call netfsqueueread() to do the queuing.

(3) Set NETFSRREQALLQUEUED independently of the queuing as we don't know the length of the subreq at this point.

(4) In all cases, set the error and NETFSSREQFAILED flag on the subreq and then call netfsreadsubreqterminated() to deal with it. This will pass responsibility off to the collector for dealing with it.