MAINTAINERS: update Geliang's email address

Update my email-address in MAINTAINERS and .mailmap entries to my
kernel.org account.

Suggested-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>