resolve_btfids: Deduplicate BTF after btf2btf transformations

btf2btf() adds new types to the BTF: the KF_IMPLICIT_ARGS transform
synthesizes an _impl FUNC together with its FUNC_PROTO and copies of the
kfunc's decl tags. Nothing deduplicates them afterwards. pahole runs
btf__dedup() on its own output, but that happens before resolve_btfids
sees the BTF, so any type the tool itself creates is emitted as-is, even
when a structurally identical type is already present.

Call btf__dedup() at the start of finalize_btf(), so that base
distillation and the by-name sort both operate on the canonical set of
types.

On an x86_64 build with the BPF selftests config this removes 17
duplicate FUNC_PROTOs from vmlinux BTF.

The dedup call increases runtime of resolve_btfids on vmlinux by 30-40%.
The performance hit is an acceptable cost to keep kernel BTF deduped [1].

[1] https://lore.kernel.org/bpf/986e6f4e-4b51-4440-a37c-9624906d7370@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://patch.msgid.link/20260807032029.78092-2-ihor.solodrai@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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