selftests/bpf: Add verifier test exercising jit PROBE_MEM logic

This patch adds a test exercising logic that was fixed / improved in
the previous patch in the series, as well as general sanity checking for
jit's PROBE_MEM logic which should've been unaffected by the previous
patch.

The added verifier test does the following:

  * Acquire a referenced kptr to struct prog_test_ref_kfunc using
    existing net/bpf/test_run.c kfunc
    * Helper returns ptr to a specific prog_test_ref_kfunc whose first
      two fields - both ints - have been prepopulated w/ vals 42 and
      108, respectively
  * kptr_xchg the acquired ptr into an arraymap
  * Do a direct map_value load of the just-added ptr
    * Goal of all this setup is to get an unreferenced kptr pointing to
      struct with ints of known value, which is the result of this step
  * Using unreferenced kptr obtained in previous step, do loads of
    prog_test_ref_kfunc.a (offset 0) and .b (offset 4)
  * Then incr the kptr by 8 and load prog_test_ref_kfunc.a again (this
    time at offset -8)
  * Add all the loaded ints together and return

Before the PROBE_MEM fixes in previous patch, the loads at offset 0 and
4 would succeed, while the load at offset -8 would incorrectly fail
runtime check emitted by the JIT and 0 out dst reg as a result. This
confirmed by retval of 150 for this test before previous patch - since
second .a read is 0'd out - and a retval of 192 with the fixed logic.

The test exercises the two optimizations to fixed logic added in last
patch as well:

  * First load, with insn "r8 = *(u32 *)(r9 + 0)" exercises "insn->off
    is 0, no need to add / sub from src_reg" optimization
  * Third load, with insn "r9 = *(u32 *)(r9 - 8)" exercises "src_reg ==
    dst_reg, no need to restore src_reg after load" optimization

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221216214319.3408356-2-davemarchevsky@fb.com

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