CVE-2026-64067: netfs: Fix missing barriers when accessing stream->subrequests locklessly

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

netfs: Fix missing barriers when accessing stream->subrequests locklessly

The list of subrequests attached to stream->subrequests is accessed without locks by netfscollectreadresults() and netfscollectwriteresults(), and then they access subreq->flags without taking a barrier after getting the subreq pointer from the list. Relatedly, the functions that build the list don't use any sort of write barrier when constructing the list to make sure that the NETFSSREQINPROGRESS flag is perceived to be set first if no lock is taken.

Fix this by:

(1) Add a new listaddtailrelease() function that uses a release barrier to set the pointer to the new member of the list.

(2) Add a new listfirstentryornullacquire() function that uses an acquire barrier to read the pointer to the first member in a list (or return NULL).

(3) Use listaddtailrelease() when adding a subreq to ->subrequests.

(4) Use listfirstentryornullacquire() when initially accessing the front of the list (when an item is removed, the pointer to the new front iterm is obtained under the same lock).