arm64: reject prctl(PR_PAC_RESET_KEYS) on compat tasks

It doesn't make sense to issue prctl(PR_PAC_RESET_KEYS) on a
compat task because the 32-bit instruction set does not offer PAuth
instructions. For consistency with other 64-bit only prctls such as
{SET,GET}_TAGGED_ADDR_CTRL, reject the prctl on compat tasks.

Although this is a userspace-visible change, maybe it isn't too late
to make this change given that the hardware isn't available yet and
it's very unlikely that anyone has 32-bit software that actually
depends on this succeeding.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/Ie885a1ff84ab498cc9f62d6451e9f2cfd4b1d06a
Link: https://lore.kernel.org/r/20201014052430.11630-1-pcc@google.com
[will: Do the same for the SVE prctl()s]
Signed-off-by: Will Deacon <will@kernel.org>