tools/bpftool: Fix PID fetching with a lot of results

In case of having so many PID results that they don't fit into a singe page
(4096) bytes, bpftool will erroneously conclude that it got corrupted data due
to 4096 not being a multiple of struct pid_iter_entry, so the last entry will
be partially truncated. Fix this by sizing the buffer to fit exactly N entries
with no truncation in the middle of record.

Fixes: d53dee3f ("tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201204232002.3589803-1-andrii@kernel.org