fs/splice: Use pipe_buf() helper to retrieve pipe buffer

Use pipe_buf() helper to retrieve the pipe buffer throughout the file
replacing the open-coded the logic.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lore.kernel.org/r/20250307052919.34542-5-kprateek.nayak@amd.com


Signed-off-by: Christian Brauner <brauner@kernel.org>