s390/fpu: make kernel fpu context preemptible

Make the kernel fpu context preemptible. Add another fpu structure to the
thread_struct, and use it to save and restore the kernel fpu context if its
task uses fpu registers when it is preempted.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>