mailmap: add entry for John Garry

get_maintainers.pl sometimes suggests my name and old e-mail address, so
update .mailmap to point to my current e-mail address.

Link: https://lkml.kernel.org/r/20240503085736.3354268-1-john.g.garry@oracle.com
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reported-by: Conor Dooley <conor@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>