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.

Fixes: 8c09871a ("s390/fpu: limit save and restore to used registers")
Cc: stable@vger.kernel.org
Reported-by: Aristeu Rozanski <aris@redhat.com>
Tested-by: Aristeu Rozanski <aris@redhat.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>