smb3: Avoid Mid pending list corruption

When reconnect happens Mid queue can be corrupted when both
demultiplex and offload thread try to dequeue the MID from the
pending list.

These patches address a problem found during decryption offload:
         CIFS: VFS: trying to dequeue a deleted mid
that could cause a refcount use after free:
         Workqueue: smb3decryptd smb2_decrypt_offload [cifs]

Signed-off-by: Rohith Surabattula <rohiths@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org> #5.4+
Signed-off-by: Steve French <stfrench@microsoft.com>