splice.2: Use 'off64_t' instead of 'loff_t'

The kernel syscall uses 'loff_t', but the glibc wrapper uses 'off64_t'.
Let's document the wrapper prototype, as in other pages.

......

.../glibc$ grep_glibc_prototype splice
sysdeps/unix/sysv/linux/bits/fcntl-linux.h:398:
extern __ssize_t splice (int __fdin, __off64_t *__offin, int __fdout,
			 __off64_t *__offout, size_t __len,
			 unsigned int __flags);
.../glibc$

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/splice.2

@@ -31,8 +31,8 @@ splice \- splice data to/from a pipe
 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
 .B #include <fcntl.h>
 .PP
-.BI "ssize_t splice(int " fd_in ", loff_t *" off_in ", int " fd_out ,
-.BI "               loff_t *" off_out ", size_t " len \
+.BI "ssize_t splice(int " fd_in ", off64_t *" off_in ", int " fd_out ,
+.BI "               off64_t *" off_out ", size_t " len \
 ", unsigned int " flags );
 .\" Return type was long before glibc 2.7
 .fi