MAINTAINERS/MEMORY MANAGEMENT: add tools/vm/ as managed files

MAINTAINERS/MEMORY MANAGEMENT: add tools/vm/ as managed files

'tools/vm/' directory should be a part of memory management subsystem, but
MAINTAINERS file doesn't mark the directory so.  Add one more 'F:' entry
for the directory to 'MEMORY MANAGEMENT' section.

Link: https://lkml.kernel.org/r/20230103180754.129637-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>