selftests: mm: skip charge_reserved_hugetlb without killall

selftests: mm: skip charge_reserved_hugetlb without killall

charge_reserved_hugetlb.sh tears down background writers with killall from
psmisc.  Minimal Ubuntu images do not always provide that tool, so the
selftest fails in cleanup for an environment reason rather than for the
hugetlb behavior it is trying to cover.

Skip the test when killall is unavailable, similar to the existing root
check, so these environments report the dependency clearly instead of
failing the test.

Link: https://lore.kernel.org/20260410044139.67480-1-create0818@163.com
Signed-off-by: Cao Ruichuang <create0818@163.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>