s390/vdso: fix stack corruption

The kernel provided vdso functions do not get a stack frame from the
calling function and therefore may not change the stack contents, unless
they allocate space on their own.

This problem was exposed with 070b7be6 "s390/vdso: replace stck with
stcke" which writes 16 bytes instead of 8 bytes into the stack frame. These
additional 8 bytes however were indeed used by the caller (glibc) to save
data and therefore this data was corrupted by the vdso code.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>