bpf: arraymap: move checks out of alloc function

Use the new callback to perform allocation checks for array maps.
The fd maps don't need a special allocation callback, they only
need a special check callback.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>