Change of license from GPLv2 to dual-license (GPLv2 and BSD 2-Clause)

All contributors were contacted off-list and permission to make this
change was received.  The complete list of contributors are Cc:ed here.

Cc: Tejun Heo <tj@kernel.org>
Cc: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Cc: Moni Shoua <monis@mellanox.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Li RongQing <roy.qing.li@gmail.com>
Cc: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Devesh Sharma <devesh.sharma@avagotech.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>