Update Linux kernel version in tst-mman-consts.py.

Update Linux kernel version in tst-mman-consts.py.

This patch updates the Linux kernel version in tst-mman-consts.py to
4.20 (meaning that's the version for which glibc is expected to have
the same constants as the kernel, up to the exceptions listed in the
test).  (Once we have more such tests sharing common infrastructure, I
expect the kernel version will be something set in the infrastructure
shared by all such tests, rather than something needing updating
separately for each test for each new kernel version.)

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/tst-mman-consts.py (main): Expect
	constants to match with Linux 4.20.