s390/mm: Change comment and die() message if teid.b61 is zero

s390/mm: Change comment and die() message if teid.b61 is zero

The comments in do_protection() give the impression that a TEID, where bit
61 is zero, indicates a low address protection exception. This is not
necessarily true, and it depends on the type of Suppression-on-Protection
facility of the machine (see Princples of Operation) what this means.

Rework the comments and the die() message to reflect this. This may also
help to avoid confusion.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>