MIPS: Properly disable FPU in start_thread()

start_thread() (called for execve(2)) clears the TIF_USEDFPU flag
without atomically disabling the FPU. With a preemptive kernel, an
unfortunately timed preemption after this could result in another
task (or KVM guest) being scheduled in with the FPU still enabled, since
lose_fpu_inatomic() only turns it off if TIF_USEDFPU is set.

Use lose_fpu(0) instead of the separate FPU / MSA management, which
should do the right thing (drop FPU properly and atomically without
saving state) and will be more future proof.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12302/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>