x86: bpf_jit: support negative offsets

Commit a998d434 claimed to introduce negative offset support to x86 jit,
but it couldn't be working, since at the time of the execution
of LD+ABS or LD+IND instructions via call into
bpf_internal_load_pointer_neg_helper() the %edx (3rd argument of this func)
had junk value instead of access size in bytes (1 or 2 or 4).

Store size into %edx instead of %ecx (what original commit intended to do)

Fixes: a998d434 ("bpf jit: Let the x86 jit handle negative offsets")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Jan Seiffert <kaffeemonster@googlemail.com>
Cc: Eric Dumazet <edumazet@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>