Update kernel version to 6.9 in header constant tests

This patch updates the kernel version in the tests tst-mman-consts.py
and tst-mount-consts.py to 6.9.  (There are no new constants covered
by these tests in 6.9 that need any other header changes;
tst-pidfd-consts.py was updated separately along with adding new
constants relevant to that test.)

Tested with build-many-glibcs.py.