CREDITS/mailmap: add some info about Darrick J. Wong

Add myself to CREDITS because apparently I've never done that; and
update mailmap so that all my old email addresses get remapped to the
kernel.org redirector.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>