fs/vfs: Fix after recent changes. write() was return negative values in errno. Noted by Jussi Kivilinna.

fs/vfs/fs_write.c

@@ -246,7 +246,7 @@ ssize_t write(int fd, FAR const void *buf, size_t nbytes)
   ret = nx_write(fd, buf, nbytes);
   if (ret < 0)
     {
-      set_errno(ret);
+      set_errno(-ret);
       ret = ERROR;
     }