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>