bpf: Reuse freed element in free_by_rcu during allocation

When there are batched freeing operations on a specific CPU, part of
the freed elements ((high_watermark - lower_watermark) / 2 + 1) will be
indirectly moved into waiting_for_gp list through free_by_rcu list.
After call_rcu_in_progress becomes false again, the remaining elements
in free_by_rcu list will be moved to waiting_for_gp list by the next
invocation of free_bulk(). However if the expiration of RCU tasks trace
grace period is relatively slow, none element in free_by_rcu list will
be moved.

So instead of invoking __alloc_percpu_gfp() or kmalloc_node() to
allocate a new object, in alloc_bulk() just check whether or not there is
freed element in free_by_rcu list and reuse it if available.

Acked-by: Yonghong Song <[email protected]>
Signed-off-by: Hou Tao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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