.mailmap: update Fangrui's email

I'm leaving Google.

Link: https://lkml.kernel.org/r/20240927192912.31532-1-i@maskray.me


Signed-off-by: Fangrui Song <i@maskray.me>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>