MAINTAINERS: update Matthieu's email address

Use my kernel.org account instead.

The other one will bounce by the end of the year.
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231004-send-net-20231004-v1-3-28de4ac663ae@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

.mailmap

@@ -377,6 +377,7 @@ Matthew Wilcox <willy@infradead.org> <willy@debian.org>
 Matthew Wilcox <willy@infradead.org> <willy@linux.intel.com>
 Matthew Wilcox <willy@infradead.org> <willy@parisc-linux.org>
 Matthias Fuchs <socketcan@esd.eu> <matthias.fuchs@esd.eu>
+Matthieu Baerts <matttbe@kernel.org> <matthieu.baerts@tessares.net>
 Matthieu CASTET <castet.matthieu@free.fr>
 Matti Vaittinen <mazziesaccount@gmail.com> <matti.vaittinen@fi.rohmeurope.com>
 Matt Ranostay <matt.ranostay@konsulko.com> <matt@ranostay.consulting>

MAINTAINERS

@@ -14942,7 +14942,7 @@ K:	macsec
 K:	\bmdo_
 
 NETWORKING [MPTCP]
-M:	Matthieu Baerts <matthieu.baerts@tessares.net>
+M:	Matthieu Baerts <matttbe@kernel.org>
 M:	Mat Martineau <martineau@kernel.org>
 L:	netdev@vger.kernel.org
 L:	mptcp@lists.linux.dev