smb: client: get rid of kstrdup() when parsing iocharset mount option

Steal string reference from @param->string rather than duplicating it.

Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>