bpf: return VMA snapshot from task_vma iterator

In the Linux kernel, the following vulnerability has been resolved:

bpf: return VMA snapshot from task_vma iterator

Holding the per-VMA lock across the BPF program body creates a lock ordering problem when helpers acquire locks that depend on mmap_lock:

vm_lock -> i_rwsem -> mmap_lock -> vm_lock

Snapshot the VMA under the per-VMA lock in _next() via memcpy(), then drop the lock before returning. The BPF program accesses only the snapshot.

The verifier only trusts vm_mm and vm_file pointers (see BTF_TYPE_SAFE_TRUSTED_OR_NULL in verifier.c). vm_file is reference- counted with get_file() under the lock and released via fput() on the next iteration or in _destroy(). vm_mm is already correct because lock_vma_under_rcu() verifies vma->vm_mm == mm. All other pointers are left as-is by memcpy() since the verifier treats them as untrusted.