mm: cachestat: fix folio read-after-free in cache walk

JIRA: https://issues.redhat.com/browse/RHEL-29132
CVE: CVE-2024-26630

This patch is a backport of the following upstream commit:
commit 3a75cb05d53f4a6823a32deb078de1366954a804
Author: Nhat Pham nphamcs@gmail.com
Date: Mon Feb 19 19:01:21 2024 -0800

mm: cachestat: fix folio read-after-free in cache walk  

In cachestat, we access the folio from the page cache's xarray to compute  
its page offset, and check for its dirty and writeback flags.  However, we  
do not hold a reference to the folio before performing these actions,  
which means the folio can concurrently be released and reused as another  
folio/page/slab.  

Get around this altogether by just using xarray's existing machinery for  
the folio page offsets and dirty/writeback states.  

This changes behavior for tmpfs files to now always report zeroes in their  
dirty and writeback counters.  This is okay as tmpfs doesn't follow  
conventional writeback cache behavior: its pages get "cleaned" during  
swapout, after which they're no longer resident etc.  

Link: https://lkml.kernel.org/r/20240220153409.GA216065@cmpxchg.org  
Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")  
Reported-by: Jann Horn <jannh@google.com>  
Suggested-by: Matthew Wilcox <willy@infradead.org>  
Signed-off-by: Nhat Pham <nphamcs@gmail.com>  
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>  
Tested-by: Jann Horn <jannh@google.com>  
Cc: <stable@vger.kernel.org>    [6.4+]  
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>  

Signed-off-by: Audra Mitchell audra@redhat.com