[PARISC] Fix show_stack() when we can't kmalloc

show_stack() was calling kzalloc() to allocate a struct pt_regs.
This meant that *really* early stack dumps would cause a null pointer
dereference.  x86_64 allocates its pt_regs on the stack, so do the same.

Kyle actually committed this exact patch to CVS on
Wed Jul 26 14:32:39 2006 UTC, and never moved it across to git.
Bad Kyle.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>