ext4: Do not fallback to buffered-io for DIO atomic write

atomic writes is currently only supported for single fsblock and only
for direct-io. We should not return -ENOTBLK for atomic writes since we
want the atomic write request to either complete fully or fail
otherwise. Hence, we should never fallback to buffered-io in case of
DIO atomic write requests.
Let's also catch if this ever happens by adding some WARN_ON_ONCE before
buffered-io handling for direct-io atomic writes. More details of the
discussion [1].

While at it let's add an inline helper ext4_want_directio_fallback() which
simplifies the logic checks and inherently fixes condition on when to return
-ENOTBLK which otherwise was always returning true for any write or directio in
ext4_iomap_end(). It was ok since ext4 only supports direct-io via iomap.

[1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04


Suggested-by: Darrick J. Wong <djwong@kernel.org> # inline helper
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>