parisc: Add error checks when building up signal trampoline handler

Add checks if the userspace trampoline code was correctly generated by the
signal trampoline generation code. In addition only flush caches as needed and
fix the old flushing code which didn't flushed all generated instructions.

Signed-off-by: Helge Deller <deller@gmx.de>

diff --cc arch/parisc/kernel/signal.c
Simple merge