selftests/mm: split_huge_page_test: skip the test when thp is not available

When thp is not enabled on some kernel config such as realtime kernel, the
test will report failure. Fix the false positive by skipping the test
directly when thp is not enabled.

Tested with thp disabled kernel:
Before The fix:
# --------------------------------------------------
# running ./split_huge_page_test /tmp/xfs_dir_Ywup9p
# --------------------------------------------------
# TAP version 13
# Bail out! Reading PMD pagesize failed
# # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
# [FAIL]
not ok 61 split_huge_page_test /tmp/xfs_dir_Ywup9p # exit=1

After the fix:
# --------------------------------------------------
# running ./split_huge_page_test /tmp/xfs_dir_YHPUPl
# --------------------------------------------------
# TAP version 13
# 1..0 # SKIP Transparent Hugepages not available
# [SKIP]
ok 6 split_huge_page_test /tmp/xfs_dir_YHPUPl # SKIP

Link: https://lore.kernel.org/20260402014543.1671131-6-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>