smb3: fix unmount hang in open_shroot

An earlier patch "CIFS: fix deadlock in cached root handling"
did not completely address the deadlock in open_shroot. This
patch addresses the deadlock.

In testing the recent patch:
  smb3: improve handling of share deleted (and share recreated)
we were able to reproduce the open_shroot deadlock to one
of the target servers in unmount in a delete share scenario.

Fixes: 7e5a70ad ("CIFS: fix deadlock in cached root handling")

This is version 2 of this patch. An earlier version of this
patch "smb3: fix unmount hang in open_shroot" had a problem
found by Dan.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

Suggested-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Aurelien Aptel <aaptel@suse.com>
CC: Stable <stable@vger.kernel.org>