bpf: change int cmd argument in __sys_bpf into typed enum bpf_cmd

bpf: change int cmd argument in __sys_bpf into typed enum bpf_cmd

This improves BTF data recorded about this function and makes
debugging/tracing better, because now command can be displayed as
symbolic name, instead of obscure number.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240905210520.2252984-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>