cifs: Skip any trailing backslashes from UNC

When extracting hostname from UNC, check for leading backslashes
before trying to remove them.

Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>