powerpc/livepatch: small cleanups in save_stack_trace_tsk_reliable()
Mostly cosmetic changes:
- Group common stack pointer code at the top
- Simplify the first frame logic
- Code stackframe iteration into for...loop construct
- Check for trace->nr_entries overflow before adding any into the array
Suggested-by: Nicolai Stange <nstange@suse.de> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>