selftests/mm: Generate a temporary mountpoint for cgroup filesystem

Currently if the filesystem for the cgroups version it wants to use is
not mounted charge_reserved_hugetlb.sh and hugetlb_reparenting_test.sh
tests will attempt to mount it on the hard coded path
/dev/cgroup/memory, deleting that directory when the test finishes. This
will fail if there is not a preexisting directory at that path, and
since the directory is deleted subsequent runs of the test will fail.
Instead of relying on this hard coded directory name use mktemp to
generate a temporary directory to use as a mountpoint, fixing both the
assumption and the disruption caused by deleting a preexisting
directory.

This means that if the relevant cgroup filesystem is not already mounted
then we rely on having coreutils (which provides mktemp) installed. I
suspect that many current users are relying on having things automounted
by default, and given that the script relies on bash it's probably not
an unreasonable requirement.

Fixes: 209376ed2a84 ("selftests/vm: make charge_reserved_hugetlb.sh work with existing cgroup setting")
Signed-off-by: Mark Brown <broonie@kernel.org>