selftests/mm: restore number of hugepages

The test mm `hugetlb_fault_after_madv` selftest needs one and only one
huge page to run, thus it sets `/proc/sys/vm/nr_hugepages` to 1.

The problem is that further tests require the previous number of hugepages
allocated in order to succeed.

Save the number of huge pages before changing it, and restore it once the
test finishes, so, further tests could run successfully.

Link: https://lkml.kernel.org/r/20231103173400.1608403-1-leitao@debian.org
Fixes: 116d5730 ("selftests/mm: add a new test for madv and hugetlb")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/all/662df57e-47f1-4c15-9b84-f2f2d587fc5c@arm.com/Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/run_vmtests.sh

@@ -223,9 +223,12 @@ CATEGORY="hugetlb" run_test ./hugepage-mremap
 CATEGORY="hugetlb" run_test ./hugepage-vmemmap
 CATEGORY="hugetlb" run_test ./hugetlb-madvise
 
+nr_hugepages_tmp=$(cat /proc/sys/vm/nr_hugepages)
 # For this test, we need one and just one huge page
 echo 1 > /proc/sys/vm/nr_hugepages
 CATEGORY="hugetlb" run_test ./hugetlb_fault_after_madv
+# Restore the previous number of huge pages, since further tests rely on it
+echo "$nr_hugepages_tmp" > /proc/sys/vm/nr_hugepages
 
 if test_selected "hugetlb"; then
 	echo "NOTE: These hugetlb tests provide minimal coverage.  Use"