bpf: Use 0 instead of NOT_INIT for btf_struct_access() writes

Returning a bpf_reg_type only makes sense in the context of a BPF_READ.
For writes, prefer to explicitly return 0 for clarity.

Note that is non-functional change as it just so happened that NOT_INIT
== 0.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/01772bc1455ae16600796ac78c6cc9fff34f95ff.1662568410.git.dxu@dxuuu.xyz


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