MAINTAINERS: update email address of Christian Borntraeger

My borntraeger@de.ibm.com email is just a forwarder to the
linux.ibm.com address. Let us remove the extra hop to avoid
a potential source of errors.

While at it, add the relevant email addresses to mailmap.

Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Link: https://lore.kernel.org/r/20211116135803.119489-2-borntraeger@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
diff --git a/.mailmap b/.mailmap
index 14314e3..6277bb2 100644
--- a/.mailmap
+++ b/.mailmap
@@ -71,6 +71,9 @@
 Chao Yu <chao@kernel.org> <yuchao0@huawei.com>
 Chris Chiu <chris.chiu@canonical.com> <chiu@endlessm.com>
 Chris Chiu <chris.chiu@canonical.com> <chiu@endlessos.org>
+Christian Borntraeger <borntraeger@linux.ibm.com> <borntraeger@de.ibm.com>
+Christian Borntraeger <borntraeger@linux.ibm.com> <cborntra@de.ibm.com>
+Christian Borntraeger <borntraeger@linux.ibm.com> <borntrae@de.ibm.com>
 Christophe Ricard <christophe.ricard@gmail.com>
 Christoph Hellwig <hch@lst.de>
 Colin Ian King <colin.king@intel.com> <colin.king@canonical.com>