cifs: fix lease break oops in xfstest generic/098

umount can race with lease break so need to check if
tcon->ses->server is still valid to send the lease
break response.

Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Fixes: 59a556ae

 ("SMB3: drop reference to cfile before sending oplock break")
Signed-off-by: Steve French <stfrench@microsoft.com>