pg_ftruncate hardcodes length=0 but only under windows

57faaf376 added pg_truncate(const char *path, off_t length), but
"length" is ignored under WIN32 and the file is unconditionally
truncated to 0.

There's no live bug, since the only caller passes 0:

| src/backend/storage/smgr/md.c:  ret = pg_truncate(path, 0);

But I guess extension users could be unhappy under win32, so maybe a fix
should be backpatched.

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index d4a46f01583..926d000f2ea 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -638,7 +638,7 @@ pg_truncate(const char *path, off_t length)
 	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
 	if (fd >= 0)
 	{
-		ret = ftruncate(fd, 0);
+		ret = ftruncate(fd, length);
 		save_errno = errno;
 		CloseTransientFile(fd);
 		errno = save_errno;