selftests/bpf: Pass page size from userspace in map_ptr

Use ASSERT to check result but keep CHECK where format was used to
report error.

Use bpf_map__set_max_entries() to set map size dynamically from
userspace according to page size.

Zero-initialize the variable in bpf prog, otherwise it will cause
problems on some versions of Clang.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-4-yauheni.kaliuta@redhat.com