fs/direct-io: Remove linux/prefetch.h include

After commit c22198e7 ("direct-io: remove random prefetches"), Nothing
in this file needs anything from `linux/prefetch.h`.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Link: https://lore.kernel.org/r/20240603014834.45294-1-youling.tang@linux.dev


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