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: 087f757b0129 ("cifs: add shutdown support") Signed-off-by: Steve French <stfrench@microsoft.com>