add linux readahead syscall

include/fcntl.h

@@ -123,6 +123,7 @@ struct f_owner_ex {
 #define SPLICE_F_MORE 4
 #define SPLICE_F_GIFT 8
 int fallocate(int, int, off_t, off_t);
+ssize_t readahead(int, off_t, size_t);
 ssize_t vmsplice(int, const struct iovec *, size_t, unsigned);
 ssize_t splice(int, off_t *, int, off_t *, size_t, unsigned);
 #define loff_t off_t

src/linux/readahead.c

+#define _GNU_SOURCE
+#include <fcntl.h>
+#include "syscall.h"
+
+ssize_t readahead(int fd, off_t pos, size_t len)
+{
+	return syscall(SYS_readahead, fd, __SYSCALL_LL_O(pos), len);
+}