arm64/signal: Consistently invalidate the in register FP state in restore

When restoring the SVE and SME specific floating point register states we
flush the task floating point state, marking the hardware state as stale so
that preemption does not result in us saving register state from the signal
handler on top of the restored context and forcing a reload from memory.
For the plain FPSIMD state we don't do this, we just copy the state from
userspace and then force an immediate reload of the register state.
This isn't racy against context switch since we copy the incoming data
onto the stack rather than directly into the task struct but it's still
messy and inconsistent.

Simplify things and avoid a potential source of error by moving the
invalidation of the CPU state to the main restore_sigframe() and
reworking the restore of the FPSIMD state to update the task struct and
rely on loading as part of the general do_notify_resume() handling for
return to user like we do for the SVE and SME state.

Signed-off-by: Mark Brown <broonie@kernel.org>