bpf: simplify verifier register state assignments

verifier is using the following structure to track the state of registers:
struct reg_state {
    enum bpf_reg_type type;
    union {
        int imm;
        struct bpf_map *map_ptr;
    };
};
and later on in states_equal() does memcmp(&old->regs[i], &cur->regs[i],..)
to find equivalent states.
Throughout the code of verifier there are assignements to 'imm' and 'map_ptr'
fields and it's not obvious that most of the assignments into 'imm' don't
need to clear extra 4 bytes (like mark_reg_unknown_value() does) to make sure
that memcmp doesn't go over junk left from 'map_ptr' assignment.

Simplify the code by converting 'int' into 'long'

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