bpf: minor cleanups for dev_map

Some minor code cleanups, while going over it I also noticed that
we're accounting the bitmap only for one CPU currently, so fix that
up as well.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>