selftests/bpf: Extend crypto_sanity selftest with invalid dst buffer

selftests/bpf: Extend crypto_sanity selftest with invalid dst buffer

Small cleanup and test extension to probe the bpf_crypto_{encrypt,decrypt}()
kfunc when a bad dst buffer is passed in to assert that an error is returned.

Also, encrypt_sanity() and skb_crypto_setup() were explicit to set the global
status variable to zero before any test, so do the same for decrypt_sanity().
Do not explicitly zero the on-stack err before bpf_crypto_ctx_create() given
the kfunc is expected to do it internally for the success case.

Before kernel fix:

  # ./vmtest.sh -- ./test_progs -t crypto
  [...]
  [    1.531200] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.533388] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #87/1    crypto_basic/crypto_release:OK
  #87/2    crypto_basic/crypto_acquire:OK
  #87      crypto_basic:OK
  test_crypto_sanity:PASS:skel open 0 nsec
  test_crypto_sanity:PASS:ip netns add crypto_sanity_ns 0 nsec
  test_crypto_sanity:PASS:ip -net crypto_sanity_ns -6 addr add face::1/128 dev lo nodad 0 nsec
  test_crypto_sanity:PASS:ip -net crypto_sanity_ns link set dev lo up 0 nsec
  test_crypto_sanity:PASS:open_netns 0 nsec
  test_crypto_sanity:PASS:AF_ALG init fail 0 nsec
  test_crypto_sanity:PASS:if_nametoindex lo 0 nsec
  test_crypto_sanity:PASS:skb_crypto_setup fd 0 nsec
  test_crypto_sanity:PASS:skb_crypto_setup 0 nsec
  test_crypto_sanity:PASS:skb_crypto_setup retval 0 nsec
  test_crypto_sanity:PASS:skb_crypto_setup status 0 nsec
  test_crypto_sanity:PASS:create qdisc hook 0 nsec
  test_crypto_sanity:PASS:make_sockaddr 0 nsec
  test_crypto_sanity:PASS:attach encrypt filter 0 nsec
  test_crypto_sanity:PASS:encrypt socket 0 nsec
  test_crypto_sanity:PASS:encrypt send 0 nsec
  test_crypto_sanity:FAIL:encrypt status unexpected error: -5 (errno 95)
  #88      crypto_sanity:FAIL
  Summary: 1/2 PASSED, 0 SKIPPED, 1 FAILED

After kernel fix:

  # ./vmtest.sh -- ./test_progs -t crypto
  [...]
  [    1.540963] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.542404] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #87/1    crypto_basic/crypto_release:OK
  #87/2    crypto_basic/crypto_acquire:OK
  #87      crypto_basic:OK
  #88      crypto_sanity:OK
  Summary: 2/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://lore.kernel.org/r/20250829143657.318524-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>