CVE-2026-31463: iomap: fix invalid folio access when i_blkbits differs from I/O granularity

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

iomap: fix invalid folio access when iblkbits differs from I/O granularity

Commit aa35dd5cbc06 ("iomap: fix invalid folio access after folioendread()") partially addressed invalid folio access for folios without an ifs attached, but it did not handle the case where 1 << inode->iblkbits matches the folio size but is different from the granularity used for the IO, which means IO can be submitted for less than the full folio for the !ifs case.

In this case, the condition:

if (bytessubmitted == foliolen) ctx->curfolio = NULL;

in iomapreadfolioiter() will not invalidate ctx->curfolio, and iomapreadend() will still be called on the folio even though the IO helper owns it and will finish the read on it.

Fix this by unconditionally invalidating ctx->curfolio for the !ifs case.