bpf: fix integer overflows

There were various issues related to the limited size of integers used in
the verifier:
 - `off + size` overflow in __check_map_access()
 - `off + reg->off` overflow in check_mem_access()
 - `off + reg->var_off.value` overflow or 32-bit truncation of
   `reg->var_off.value` in check_mem_access()
 - 32-bit truncation in check_stack_boundary()

Make sure that any integer math cannot overflow by not allowing
pointer math with large values.

Also reduce the scope of "scalar op scalar" tracking.

Fixes: f1174f77

 ("bpf/verifier: rework value tracking")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>