smb3: fix problem remounting a share after shutdown

xfstests generic/392 showed a problem where even after a
shutdown call was made on a mount, we would still attempt
to use the (now inaccessible) superblock if another mount
was attempted for the same share.

Reported-by: David Howells <dhowells@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Fixes: 087f757b ("cifs: add shutdown support")
Signed-off-by: Steve French <stfrench@microsoft.com>