selftests/bpf/test_offload.py: Filter bpftool internal map when counting maps

A few of the tests in test_offload.py expects to see a certain number of
maps created, and checks this by counting the number of maps returned by
bpftool. There is already a filter that will remove any maps already there
at the beginning of the test, but bpftool now creates a map for the PID
iterator rodata on each invocation, which makes the map count wrong. Fix
this by also filtering the pid_iter.rodata map by name when counting.

Fixes: d53dee3f ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752226387.110217.9887866138149423444.stgit@toke.dk