Move all arguments into output registers from input registers.

This path is exercised by test_verifier.c's "calls: two calls with
args" test.  Adjust BPF_TAILCALL_PROLOGUE_SKIP as needed.

Let's also make the prologue length a constant size regardless of
the combination of ->saw_frame_pointer and ->saw_tail_call
settings.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>