ext4: Check for atomic writes support in write iter

Let's validate the given constraints for atomic write request.
Otherwise it will fail with -EINVAL. Currently atomic write is only
supported on DIO, so for buffered-io it will return -EOPNOTSUPP.

Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>