arm64/fpsimd: Ensure SME storage is allocated after SVE VL changes

When we reconfigure the SVE vector length we discard the backing storage
for the SVE vectors and then reallocate on next SVE use, leaving the SME
specific state alone. This means that we do not enable SME traps if they
were already disabled. That means that userspace code can enter streaming
mode without trapping, putting the task in a state where if we try to save
the state of the task we will fault.

Since the ABI does not specify that changing the SVE vector length disturbs
SME state, and since SVE code may not be aware of SME code in the process,
we shouldn't simply discard any ZA state. Instead immediately reallocate
the storage for SVE, and disable SME if we change the SVE vector length
while there is no SME state active.

Disabling SME traps on SVE vector length changes would make the overall
code more complex since we would have a state where we have valid SME state
stored but might get a SME trap.

Fixes: 9e4ab6c8

 ("arm64/sme: Implement vector length configuration prctl()s")
Reported-by: David Spickett <David.Spickett@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230720-arm64-fix-sve-sme-vl-change-v2-1-8eea06b82d57@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>