.mailmap: update Fangrui's email

I'm leaving Google.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Fangrui Song <[email protected]>
Acked-by: Nathan Chancellor <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: MaskRay

.mailmap

@@ -203,6 +203,7 @@ Ezequiel Garcia <[email protected]> <[email protected]>
 Faith Ekstrand <[email protected]> <[email protected]>
 Faith Ekstrand <[email protected]> <[email protected]>
 Faith Ekstrand <[email protected]> <[email protected]>
+Fangrui Song <[email protected]> <[email protected]>
 Felipe W Damasio <[email protected]>
 Felix Kuhling <[email protected]>
 Felix Moeller <[email protected]>