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>