s390/stacktrace: Improve detection of invalid instruction pointers

Add basic checks to identify invalid instruction pointers when walking
stack frames:

Instruction pointers must

- have even addresses
- be larger than mmap_min_addr
- lower than the asce_limit of the process

Alternatively it would also be possible to walk page tables similar to fast
GUP and verify that the mapping of the corresponding page is executable,
however that seems to be overkill.

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>