splice: divorce the splice structure/function definitions from the pipe header

We need to move even more stuff into the header so that folks can use
the splice_to_pipe() implementation instead of open-coding a lot of
pipe knowledge (see relay implementation), so move to our own header
file finally.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/read_write.c b/fs/read_write.c
index 47da8a4..2527cf0 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -15,7 +15,7 @@
 #include <linux/module.h>
 #include <linux/syscalls.h>
 #include <linux/pagemap.h>
-#include <linux/pipe_fs_i.h>
+#include <linux/splice.h>
 #include "read_write.h"
 
 #include <asm/uaccess.h>