tools/vm: rename tools/vm to tools/mm

Rename tools/vm to tools/mm for being more consistent with the code and
documentation directories, and won't be confused with virtual machines.

Link: https://lkml.kernel.org/r/20230103180754.129637-4-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>