selftests/mm: alloc hugepages in va_high_addr_switch test

selftests/mm: alloc hugepages in va_high_addr_switch test

Alloc hugepages in the test internally, so we don't fully rely on the
run_vmtests.sh.  If run_vmtests.sh does that great, free hugepages is
enough for being used to run the test, leave it as it is, otherwise setup
the hugepages in the test.

Save the original nr_hugepages value and restore it after test finish, so
leave a stable test envronment.

Link: https://lkml.kernel.org/r/20250912013711.3002969-3-chuhu@redhat.com
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>