s390/uaccess: always load the kernel ASCE after task switch

This patch fixes a problem introduced with git commit beef560b
"s390/uaccess: simplify control register updates".

The switch_mm function is not called if the next process is a kernel
thread without an attached mm or is a nop if the mm does not change.
But CR1 still needs to be loaded with the kernel ASCE in case the
code returns to a uaccess function that uses the secondary space mode.

In addition move the set_fs call from finish_arch_switch to
finish_arch_post_lock_switch and then remove finish_arch_switch.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>