Merge branch 'bpf: Small nf_conn cleanups'

Daniel Xu says:

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This patchset cleans up a few small things:

* Delete unused stub
* Rename variable to be more descriptive
* Fix some `extern` declaration warnings

Past discussion:
- v2: https://lore.kernel.org/bpf/cover.1663616584.git.dxu@dxuuu.xyz/

Changes since v2:
- Remove unused #include's
- Move #include <linux/filter.h> to .c
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>