bpf: Postpone bpf_selem_free() in bpf_selem_unlink_storage_nolock()

In a later patch, bpf_selem_free() will call unpin_user_page()
through bpf_obj_free_fields(). unpin_user_page() may take spin_lock.
However, some bpf_selem_free() call paths have held a raw_spin_lock.
Like this:

raw_spin_lock_irqsave()
  bpf_selem_unlink_storage_nolock()
    bpf_selem_free()
      unpin_user_page()
        spin_lock()

To avoid spinlock nested in raw_spinlock, bpf_selem_free() should be
done after releasing the raw_spinlock. The "bool reuse_now" arg is
replaced with "struct hlist_head *free_selem_list" in
bpf_selem_unlink_storage_nolock(). The bpf_selem_unlink_storage_nolock()
will append the to-be-free selem at the free_selem_list. The caller of
bpf_selem_unlink_storage_nolock() will need to call the new
bpf_selem_free_list(free_selem_list, reuse_now) to free the selem
after releasing the raw_spinlock.

Note that the selem->snode cannot be reused for linking to
the free_selem_list because the selem->snode is protected by the
raw_spinlock that we want to avoid holding. A new
"struct hlist_node free_node;" is union-ized with
the rcu_head. Only the first one successfully
hlist_del_init_rcu(&selem->snode) will be able
to use the free_node. After succeeding hlist_del_init_rcu(&selem->snode),
the free_node and rcu_head usage is serialized such that they
can share the 16 bytes in a union.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20241023234759.860539-5-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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