selftests/bpf: Fix htab_update/reenter_update selftest failure

Since commit 31158ad02ddb ("rqspinlock: Add deadlock detection
and recovery") the updated path on re-entrancy now reports deadlock
via -EDEADLK instead of the previous -EBUSY.

Also, the way reentrancy was exercised (via fentry/lookup_elem_raw)
has been fragile because lookup_elem_raw may be inlined
(find_kernel_btf_id() will return -ESRCH).

To fix this fentry is attached to bpf_obj_free_fields() instead of
lookup_elem_raw() and:

- The htab map is made to use a BTF-described struct val with a
  struct bpf_timer so that check_and_free_fields() reliably calls
  bpf_obj_free_fields() on element replacement.

- The selftest is updated to do two updates to the same key (insert +
  replace) in prog_test.

- The selftest is updated to align with expected errno with the
  kernel’s current behavior.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/r/20251117060752.129648-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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