bpf: Clarify bpf_arena comments.

Clarify two bpf_arena comments, use existing SZ_4G #define,
improve page_cnt check.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20240315021834.62988-2-alexei.starovoitov@gmail.com