Disable FOP_DONTCACHE for now due to bugs

This is kind of last-minute, but Al Viro reported that the new
FOP_DONTCACHE flag causes memory corruption due to use-after-free
issues.

This was triggered by commit 974c5e6139db ("xfs: flag as supporting
FOP_DONTCACHE"), but that is not the underlying bug - it is just the
first user of the flag.

Vlastimil Babka suspects the underlying problem stems from the
folio_end_writeback() logic introduced in commit fb7d3bc414939
("mm/filemap: drop streaming/uncached pages when writeback completes").

The most straightforward fix would be to just revert the commit that
exposed this, but Matthew Wilcox points out that other filesystems are
also starting to enable the FOP_DONTCACHE logic, so this instead
disables that bit globally for now.

The fix will hopefully end up being trivial and we can just re-enable
this logic after more testing, but until such a time we'll have to
disable the new FOP_DONTCACHE flag.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/all/20250525083209.GS2023217@ZenIV/
Triggered-by: 974c5e6139db ("xfs: flag as supporting FOP_DONTCACHE")
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>