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

selftests/powerpc: Use date instead of EPOCHSECONDS in mitigation-patching.sh

The EPOCHSECONDS environment variable was added in bash 5.0 (released
2019). Some distributions of the "stable" and "long-term" variety ship
older versions of bash than this, so swap to using the date command
instead.

"%s" was added to coreutils `date` in 1993 so we should be good, but who
knows, it is a GNU extension and not part of the POSIX spec for `date`.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211025102436.19177-1-ruscur@russell.cc

selftests/powerpc: Add test of mitigation patching

We recently discovered some of our mitigation patching was not safe
against other CPUs running concurrently.

Add a test which enable/disables all mitigations in a tight loop while
also running some stress load. On an unpatched system this almost always
leads to an oops and panic/reboot, but we also check if the kernel
becomes tainted in case we have a non-fatal oops.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210507064225.1556312-1-mpe@ellerman.id.au