[5.15] Please apply 'selftest/lkdtm: Skip stack-entropy test if lkdtm is not available'

[Andrew Paniakin]

commit 90091c367e74d5b58d9ebe979cc363f7468f58d3 upstream.

This patch fixes the stack-entropy.sh test to exit gracefully when the LKDTM is
not available. Test will hang otherwise as reported in [1].

Applicability of this fix to other LTS kernels:
- 4.14: No lkdtm selftest
- 4.19: No lkdtm selftest
- 5.4:  No lkdtm selftests
- 5.10: Inital selftest version introduced in 46d1a0f03d661 ("selftests/lkdtm:
  Add tests for LKDTM targets") is a single script which has the LKDTM
  availability check
- 6.1: Fix applied

This patch applies cleanly to stable-5.15 tree. Updated test was executed in
Qemu VM with different kernels:
- CONFIG_LKDTM not enabled. Test finished with status SKIP.
- CONFIG_LKDTM enabled. Test failed (but not hanged) with error 'Stack entropy
  is low'.
- CONFIG_LKDTM enabled and randomize_kstack_offset=on boot argument provided.
  Test succeed.

[1] https://lore.kernel.org/lkml/2836f48a-d4e2-7f00-f06c-9f556fbd6332@xxxxxxxxxxxxxxxxxxx