s390/mmap: take stack_guard_gap into account for mmap_base

The s390 version of the mmap_base function is ignorant of stack_guard_gap
which can lead to a placement of the stack vs. the mmap base that does not
leave enough space for the stack rlimit.

Add the stack_guard_gap to the calculation and while we are at it the
check for gap+pad overflows as well.

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