mailmap: update Kirill's email

My new email address is kirill.tkhai@openvz.org.

Link: https://lkml.kernel.org/r/164846762354.278960.13129571556274098855.stgit@pro


Signed-off-by: Kirill Tkhai <kirill.tkhai@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>