mailmap: update Seth Forshee's email address

seth.forshee@canonical.com is no longer valid, use sforshee@kernel.org
instead.

Link: https://lkml.kernel.org/r/20220628200734.424495-1-sforshee@kernel.org


Signed-off-by: Seth Forshee <sforshee@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>