chacha20poly1305: use slow crypto on -rt kernels

In rt kernels, spinlocks call schedule(), which means preemption can't
be disabled. The FPU disables preemption. Hence, we can either
restructure things to move the calls to kernel_fpu_begin/end to be
really close to the actual crypto routines, or we can do the slower
lazier solution of just not using the FPU at all on -rt kernels. This
patch goes with the latter lazy solution.

The reason why we don't place the calls to kernel_fpu_begin/end close to
the crypto routines in the first place is that they're very expensive,
as it usually involves a call to XSAVE. So on sane kernels, we benefit
from only having to call it once.

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