llc: fix misaligned MetaInfo access in msgb headroom
struct MetaInfo (two struct timespec) requires 8-byte alignment on some ABIs (e.g. 32-bit ARM with a 64-bit time_t), but msgb->head is only guaranteed byte alignment: struct msgb's own size before the flexible _data[] member is not a multiple of 8, so storing MetaInfo directly at msg->head can misalign it, triggering UBSan -fsanitize=alignment reports wherever recv_time/expire_time are subsequently dereferenced.
Reserve extra headroom for worst-case alignment padding and store/retrieve MetaInfo via the next 8-byte-aligned address within that headroom instead of assuming msg->head itself is suitably aligned.
