Since that version, the FPU is only updated when returning to userspace.
This breaks the thread switching expectations of Xenomai because there
is no update happening when returning to a thread in primary mode or
when switching in a primary-mode kernel thread.

Account for that by
 - saving the context when switching out an in-kernel Xenomai thread
 - restoring the context after switching in a Xenomai thread
 - invalidate the FPU state variables after returning to a preempted
   Linux kernel thread that uses the FPU

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>