bpf: fix shift upon scatterlist ring wrap-around in bpf_msg_pull_data

If first_sg and last_sg wraps around in the scatterlist ring, then we
need to account for that in the shift as well. E.g. crafting such msgs
where this is the case leads to a hang as shift becomes negative. E.g.
consider the following scenario:

  first_sg := 14     |=>    shift := -12     msg->sg_start := 10
  last_sg  :=  3     |                       msg->sg_end   :=  5

round  1:  i := 15, move_from :=   3, sg[15] := sg[  3]
round  2:  i :=  0, move_from := -12, sg[ 0] := sg[-12]
round  3:  i :=  1, move_from := -11, sg[ 1] := sg[-11]
round  4:  i :=  2, move_from := -10, sg[ 2] := sg[-10]
[...]
round 13:  i := 11, move_from :=  -1, sg[ 2] := sg[ -1]
round 14:  i := 12, move_from :=   0, sg[ 2] := sg[  0]
round 15:  i := 13, move_from :=   1, sg[ 2] := sg[  1]
round 16:  i := 14, move_from :=   2, sg[ 2] := sg[  2]
round 17:  i := 15, move_from :=   3, sg[ 2] := sg[  3]
[...]

This means we will loop forever and never hit the msg->sg_end condition
to break out of the loop. When we see that the ring wraps around, then
the shift should be MAX_SKB_FRAGS - first_sg + last_sg - 1. Meaning,
the remainder slots from the tail of the ring and the head until last_sg
combined.

Fixes: 015632bb ("bpf: sk_msg program helper bpf_sk_msg_pull_data")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>