selftests: bpf: Add a test when bpf_probe_read_kernel_str() returns EFAULT

This commit tests previous fix of bpf_probe_read_kernel_str().

The BPF helper bpf_probe_read_kernel_str should return -EFAULT when
given a bad source pointer and the target buffer should only be modified
to make the string NULL terminated.

bpf_probe_read_kernel_str() was previously inserting a NULL before the
beginning of the dst buffer. This test should ensure that the
implementation stays correct for now on.

Without the fix, this test will fail as follows:
  $ cd tools/testing/selftests/bpf
  $ make
  $ sudo ./test_progs --name=varlen
  ...
  test_varlen:FAIL:check got 0 != exp 66

Signed-off-by: Alban Crequy <[email protected]>
Signed-off-by: Francis Laniel <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Changes v1 to v2:
- add ack tag
- fix my email
- rebase on bpf tree and tag for bpf tree

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