fs: Replace offsetof() with struct_size() in ioctl_file_dedupe_range()

When dealing with structures containing flexible arrays, struct_size()
provides additional compile-time checks compared to offsetof(). This
enhances code robustness and reduces the risk of potential errors.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Link: https://lore.kernel.org/20250829091510.597858-1-zhao.xichao@vivo.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>