bpf: Fix remap of arena.

The bpf arena logic didn't account for mremap operation. Add a refcnt for
multiple mmap events to prevent use-after-free in arena_vm_close.

Fixes: 317460317a02 ("bpf: Introduce bpf_arena.")
Reported-by: Pengfei Xu <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Barret Rhoden <[email protected]>
Tested-by: Pengfei Xu <[email protected]>
Closes: https://lore.kernel.org/bpf/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]

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