powerpc/dexcr: Support userspace ROP protection

The ISA 3.1B hashst and hashchk instructions use a per-cpu SPR HASHKEYR
to hold a key used in the hash calculation. This key should be different
for each process to make it harder for a malicious process to recreate
valid hash values for a victim process.

Add support for storing a per-thread hash key, and setting/clearing
HASHKEYR appropriately.

Signed-off-by: Benjamin Gray <[email protected]>
Reviewed-by: Russell Currey <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

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