mm: memblock: add missing HugeTLB flag name

Commit 7d163a75f821 ("memblock: make HugeTLB bootmem allocation work
with KHO") added MEMBLOCK_RSRV_HUGETLB but did not add the corresponding
entry to flagname[]. As a result, memblock debugfs cannot report the
flag by name.

Add the missing RSV_HUGETLB entry.

Fixes: 7d163a75f821 ("memblock: make HugeTLB bootmem allocation work with KHO")
Signed-off-by: Meijing Zhao <zhaomeijing@lixiang.com>
Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
Link: https://lore.kernel.org/lkml/20260821020910.3428585-2-zhaomeijing100@gmail.com/
Link: https://patch.msgid.link/20260915124641.2498280-1-chenwandun1@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>