s390/fpu: Re-add exception handling in load_fpu_state()

In the Linux kernel, the following vulnerability has been resolved: s390/fpu: Re-add exception handling in load_fpu_state() With the recent rewrite of the fpu code exception handling for the lfpc instruction within load_fpu_state() was erroneously removed. Add it again to prevent that loading invalid floating point register values cause an unhandled specification exception.