x86: be even more careful about checking the stack frame on dumping

lguest didn't initialize the kernel stack the way a real i386 kernel
does, and ended up triggering a corner-case in the stack frame checking
that doesn't happen on naive i386, and that the stack dumping didn't
handle quite right.

This makes the frame handling more correct, and tries to clarify the
code at the same time so that it's a bit more obvious what is going on.

Thanks to Rusty Russell for debugging the lguest failure-

Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>