Introduce and use struct stack_frame_vdso_wrapper within vdso user wrapper
code.  With this structure it is possible to automatically generate an
asm-offset define which can be used to save and restore the return address
of the calling function.

Also use STACK_FRAME_USER_OVERHEAD instead of STACK_FRAME_OVERHEAD to
document that the code works with user space stack frames with the standard
stack frame layout.

Fixes: aa44433a ("s390: add USER_STACKTRACE support")
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>