selftests/mm: transhuge_stress: skip the test when thp not available

The test requires thp, skip the test when thp is not available to avoid
false positive.

Tested with thp disabled kernel.
Before the fix:
# --------------------------------
# running ./transhuge-stress -d 20
# --------------------------------
# TAP version 13
# 1..1
# transhuge-stress: allocate 1453 transhuge pages, using 2907 MiB virtual memory and 11 MiB of ram
# Bail out! MADV_HUGEPAGE# Planned tests != run tests (1 != 0)
# # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
# [FAIL]
not ok 60 transhuge-stress -d 20 # exit=1

After the fix:
# --------------------------------
# running ./transhuge-stress -d 20
# --------------------------------
# TAP version 13
# 1..0 # SKIP Transparent Hugepages not available
# [SKIP]
ok 5 transhuge-stress -d 20 # SKIP

Link: https://lore.kernel.org/20260402014543.1671131-7-chuhu@redhat.com
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Li Wang <liwang@redhat.com>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>