splice: fix unexpected size truncation

@splice_desc.total_len is 32 bit(unsigned int) which is used to store the
size passed from userspace which is 64 bit(size_t) so that the size is
unexpectedly truncated

That means vmsplice can not work if the size passed from userspace is >=
4G, for example, we noticed in vmsplice, splice-reader does not do
anything and splice-writer is waiting for available buffer forever if the
size is 4G

Fix it by extending @splice_desc.total_len to 64 bits as well

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/splice.h

@@ -24,7 +24,8 @@
  * Passed to the actors
  */
 struct splice_desc {
-	unsigned int len, total_len;	/* current and remaining length */
+	size_t total_len;		/* remaining length */
+	unsigned int len;		/* current length */
 	unsigned int flags;		/* splice flags */
 	/*
 	 * actor() private data