mailmap: add Changbin Du

Add my email in the mailmap file to have a consistent shortlog output.

Link: http://lkml.kernel.org/r/20190308142103.4929-1-changbin.du@gmail.com


Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>