bpf: refactor btf_find_struct_field() and btf_find_datasec_var().

Move common code of the two functions to btf_find_field_one().

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240523174202.461236-4-thinker.li@gmail.com


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