fuse: fsync() did not return IO errors

commit ac7f052b upstream.

Due to implementation of fuse writeback filemap_write_and_wait_range() does
not catch errors. We have to do this directly after fuse_sync_writes()

Signed-off-by: Alexey Kuznetsov <kuznet@virtuozzo.com>
Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 4d99ff8f ("fuse: Turn writeback cache on")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>