[S390] switch_to: dont restore/save access & fpu regs for kernel threads

If the previous task was a kernel thread there is no need to save the
contents of the fpu and access registers since they aren't used in
kernel mode.
For the same reason it is not necessary to restore these registers if
the next task is a kernel thread.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>