maps: Handle GetValueAndDeleteBatch return code appropiately

[javierhonduco]

Long story short, some libbpf APIs that don't return pointers, such as the
batch APIs might return -1 when there's a 'partial success'. In the case
of these APIs, with errno=-ENOENT, we know that we read and deleted less than
count entries, but it was still overall sucessful.

This PR fixes this for GetValueAndDeleteBatch only. We aren't addressing
all the other batch calls yet as we haven't used them
yet.

Kemal originally tacked this in #152, but unfortunately
not checking the return code and errno makes the interpretation of return values
bogus. I have traced the kernel code and we never returned EFAULT at least in this
libbpf call.

Test plan

$ make run-dynamic
make -C /home/vagrant/work/libbpfgo libbpfgo-dynamic
make[1]: Entering directory '/home/vagrant/work/libbpfgo'
CC=clang \
        CGO_CFLAGS= \
        CGO_LDFLAGS="-lelf -lz -lbpf" \
        go build .
make[1]: Leaving directory '/home/vagrant/work/libbpfgo'
CC=clang \
        CGO_CFLAGS= \
        CGO_LDFLAGS="-lelf -lz -lbpf" \
        go build -o ./main-dynamic ./main.go
sudo ./run.sh main-dynamic
[*] SUCCESS: all good

Thanks!

cc/ @Sylfrena @v-thakkar @kakkoyun

[CLAassistant]

All committers have signed the CLA.

[kakkoyun]

Looking good. I think we might get away without changing the signatures.
Also, I have an inlined question.

[javierhonduco]

BTW this PR should fix #147

[grantseltzer]

This LGTM! Thanks for your help.

I'd really like to make sure each batch API is updated for correctness alongside this. Do you plan on doing so? It's alright if not, I just want to add it to my queue and keep track!

[javierhonduco]

@grantseltzer I will probably tackle the DeleteKeyBatch API, will keep you updated! Created #162 to track it