get_maintainer: Honor mailmap for in file emails

Add support to also use the mailmap for 'in file' email addresses.

Link: https://lkml.kernel.org/r/20220323193645.317514-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Reported-by: Marc Zyngier <maz@kernel.org>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>