BACKPORT: fs: add RWF_DONTCACHE iocb and FOP_DONTCACHE file_operations flag

If a file system supports uncached buffered IO, it may set FOP_DONTCACHE
and enable support for RWF_DONTCACHE.  If RWF_DONTCACHE is attempted
without the file system supporting it, it'll get errored with -EOPNOTSUPP.

Bug: 416315033
(cherry picked from commit b9f958d4)
[ akailash: Fix merge conflict in include/linux/fs.h.
 FOP_ASYNC_LOCK flag isn't present in ACK 6.12; We skip that
 and add FOP_DONTCACHE only. ]
Link: https://lkml.kernel.org/r/20241220154831.1086649-8-axboe@kernel.dk


Change-Id: Iaf9a6bf8c42ecaee419072ed6cd0f9481bafe8fd
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Brian Foster <bfoster@redhat.com>
Cc: Chris Mason <clm@meta.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Akilesh Kailash <akailash@google.com>