fs: add IOCB_SYNC and IOCB_DSYNC

This will allow us to do per-I/O sync file writes, as required by a lot
of fileservers or storage targets.

XXX: Will need a few additional audits for O_DSYNC

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/filemap.c

@@ -2794,7 +2794,7 @@ ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	if (ret > 0) {
 		ssize_t err;
 
-		err = generic_write_sync(file, iocb->ki_pos - ret, ret);
+		err = generic_write_sync(iocb, iocb->ki_pos - ret, ret);
 		if (err < 0)
 			ret = err;
 	}