tdb2: fix pread/pwrite error handling in fill and tdb_write.

The "ret < n" was done as an unsigned comparison, so it didn't work as
expected when ret was negative.

Simplest fix is to do an equals comparison everywhere, which is also
slightly stricter.

ccan/tdb2/io.c

@@ -246,7 +246,7 @@ static enum TDB_ERROR tdb_write(struct tdb_context *tdb, tdb_off_t off,
 	} else {
 		ssize_t ret;
 		ret = pwrite(tdb->fd, buf, len, off);
-		if (ret < len) {
+		if (ret != len) {
 			/* This shouldn't happen: we avoid sparse files. */
 			if (ret >= 0)
 				errno = ENOSPC;
@@ -375,7 +375,7 @@ static enum TDB_ERROR fill(struct tdb_context *tdb,
 	while (len) {
 		size_t n = len > size ? size : len;
 		ssize_t ret = pwrite(tdb->fd, buf, n, off);
-		if (ret < n) {
+		if (ret != n) {
 			if (ret >= 0)
 				errno = ENOSPC;