selftests/bpf: Test the valid bytes of a packet pointer in a counted loop

Each test adds r4 to a packet pointer. r4 is 0, 8 .. 48, but its umax is
50.

  test                                      umax gets smaller    result
  ----------------------------------------  -------------------  ------
  widen_pkt_pointer_step_lowers_umax        at the loop entry    reject
  widen_pkt_pointer_step_lowers_umax_down   the same, r6 += -8   reject
  widen_pkt_pointer_negative_offset         no, only smax        reject
  clamp_pkt_pointer_step_lowers_umax        at the back edge     reject
  union_pkt_pointer_var_off_lowers_umax     r5 is r6 + 8 or r6   reject
  widen_pkt_pointer_keeps_valid_bytes       at the loop entry    loads

The first five progs load without the fix. The last prog reads only bytes
that the compare with pkt_end covers.

widen_pkt_pointer_beyond_pkt_end makes sure that the mark "beyond pkt_end"
is not in the state at the head of the loop.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261030210950.1904640-3-alexei.starovoitov@gmail.com

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