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 <rnsastry@linux.ibm.com>
Signed-off-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/163941374362.36967.18016981579099073379.sendpatchset@1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa

tools/testing/selftests/powerpc/security/mitigation-patching.sh

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