Update kernel version to 6.8 in header constant tests

This patch updates the kernel version in the tests tst-mman-consts.py,
tst-mount-consts.py and tst-pidfd-consts.py to 6.8.  (There are no new
constants covered by these tests in 6.8 that need any other header
changes.)

Tested with build-many-glibcs.py.