Functions which end in a BUG() statement and skip the return statement
cause compile warnings on s390, e.g.:

mm/bootmem.c: In function 'mark_bootmem':
mm/bootmem.c:321: warning: control reaches end of non-void function

To avoid the warning add an endless loop to the BUG() macro.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>