MAINTAINERS: add Jann as memory mapping/VMA reviewer

Add myself as a reviewer for memory mapping / VMA code.  I will probably
only reply to patches sporadically, but hopefully this will help me keep
up with changes that look interesting security-wise.

Link: https://lkml.kernel.org/r/20241014-maintainers-mmap-reviewer-v1-1-50dce0514752@google.com


Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>