smb: client: Return directly after a failed genlmsg_new() in cifs_swn_send_register_message()

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* Return directly after a call of the function “genlmsg_new” failed
  at the beginning.

* Delete the label “fail” which became unnecessary
  with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Steve French <stfrench@microsoft.com>