[PATCH]selftests/powerpc: skip tests for unavailable mitigations.

Mitigation patching test iterates over a set of mitigations irrespective
of whether a certain mitigation is supported/available in the kernel.
This causes following messages on a kernel where some mitigations
are unavailable:

  Spawned threads enabling/disabling mitigations ...
  cat: entry_flush: No such file or directory
  cat: uaccess_flush: No such file or directory
  Waiting for timeout ...
  OK

This patch adds a check for available mitigations in the kernel.

Reported-by: Nageswara R Sastry <redacted>
Signed-off-by: Sachin Sant <redacted>
---
diff -Naurp aa/tools/testing/selftests/powerpc/security/mitigation-patching.sh bb/tools/testing/selftests/powerpc/security/mitigation-patching.sh

--- aa/tools/testing/selftests/powerpc/security/mitigation-patching.sh	2021-12-13 10:17:05.714127154 -0500
+++ bb/tools/testing/selftests/powerpc/security/mitigation-patching.sh	2021-12-13 10:19:32.575315913 -0500

@@ -44,7 +44,10 @@ mitigations="barrier_nospec stf_barrier
 
 for m in $mitigations
 do
-    do_one "$m" &
+    if [[ -f /sys/kernel/debug/powerpc/$m ]]
+    then
+        do_one "$m" &
+    fi
 done
 
 echo "Spawned threads enabling/disabling mitigations ..."