cifs: smbd: Properly process errors on ib_post_send

When processing errors from ib_post_send(), the transport state needs to be
rolled back to the condition before the error.

Refactor the old code to make it easy to roll back on IB errors, and fix this.

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>