cifs: on send failure, readjust server sequence number downward

If sending a call to the server fails for some reason (for instance, the
sending thread caught a signal), then we must readjust the sequence
number downward again or the next send will have it too high.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Steve French <smfrench@gmail.com>