s390/vdso: remove BUG_ON()

Handle allocation error gracefully and simply disable vdso instead of
leaving the system in an undefined state.

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