bpf: make an argument const in the bpf_map_sum_elem_count kfunc

We use the map pointer only to read the counter values, no locking
involved, so mark the argument as const.

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230719092952.41202-4-aspsk@isovalent.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>