s390: fix fpu restore in entry.S

s390: fix fpu restore in entry.S

We need to disable interrupts in load_fpu_regs(). Otherwise an
interrupt might come in after the registers are loaded, but before
CIF_FPU is cleared in load_fpu_regs(). When the interrupt returns,
CIF_FPU will be cleared and the registers will never be restored.

The entry.S code usually saves the interrupt state in __SF_EMPTY on the
stack when disabling/restoring interrupts. sie64a however saves the pointer
to the sie control block in __SF_SIE_CONTROL, which references the same
location.  This is non-obvious to the reader. To avoid thrashing the sie
control block pointer in load_fpu_regs(), move the __SIE_* offsets eight
bytes after __SF_EMPTY on the stack.

Cc: <[email protected]> # 5.8
Fixes: 0b0ed657fe00 ("s390: remove critical section cleanup from entry.S")
Reported-by: Pierre Morel <[email protected]>
Signed-off-by: Sven Schnelle <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Reviewed-by: Heiko Carstens <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>