liveupdate: synchronize lazy initialization of FLB private state

The luo_flb_get_private() function, which is responsible for lazily
initializing the private state of FLB objects, can be called concurrently
from multiple threads.  This creates a data race on the 'initialized' flag
and can lead to multiple executions of mutex_init() and INIT_LIST_HEAD()
on the same memory.

Introduce a static spinlock (luo_flb_init_lock) local to the function to
synchronize the initialization path.  Use smp_load_acquire() and
smp_store_release() for memory ordering between the fast path and the slow
path.

Link: https://lore.kernel.org/20260327033335.696621-3-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: David Matlack <dmatlack@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Samiullah Khawaja <skhawaja@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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