CVE-2026-23002: lib/buildid: use __kernel_read() for sleepable context

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

lib/buildid: use kernelread() for sleepable context

Prevent a "BUG: unable to handle kernel NULL pointer dereference in filemapreadfolio".

For the sleepable context, convert freader to use kernelread() instead of direct page cache access via readcachefolio(). This simplifies the faultable code path by using the standard kernel file reading interface which handles all the complexity of reading file data.

At the moment we are not changing the code for non-sleepable context which uses filemapgetfolio() and only succeeds if the target folios are already in memory and up-to-date. The reason is to keep the patch simple and easier to backport to stable kernels.

Syzbot repro does not crash the kernel anymore and the selftests run successfully.

In the follow up we will make kernelread() with IOCBNOWAIT work for non-sleepable contexts. In addition, I would like to replace the secretmem check with a more generic approach and will add fstest for the buildid code.