Merge branch 'Don't reinit map value in prealloc_lru_pop'

Kumar Kartikeya Dwivedi says:

====================

Fix for a bug in prealloc_lru_pop spotted while reading the code, then a test +
example that checks whether it is fixed.

Changelog:
----------
v2 -> v3:
v2: https://lore.kernel.org/bpf/20220809140615.21231-1-memxor@gmail.com

 * Switch test to use kptr instead of kptr_ref to stabilize test runs
 * Fix missing lru_bug__destroy (Yonghong)
 * Collect Acks

v1 -> v2:
v1: https://lore.kernel.org/bpf/20220806014603.1771-1-memxor@gmail.com

 * Expand commit log to include summary of the discussion with Yonghong
 * Make lru_bug selftest serial to not mess up refcount for map_kptr test
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>