bpf: htab: extract helper for freeing special structs

Extract the cleanup of known embedded structs into the dedicated helper.
Remove duplication and introduce a single source of truth for freeing
special embedded structs in hashtab.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250923112404.668720-4-mykyta.yatsenko5@gmail.com


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