sh: unwinder: Convert frame allocations to GFP_ATOMIC.

save_stack_trace_tsk() and friends can be called from atomic context (as
triggered by latencytop), and subsequently hit two problematic allocation
points that were using GFP_KERNEL (these were dwarf_unwind_stack() and
dwarf_frame_alloc_regs()). Convert these over to GFP_ATOMIC and get
latencytop working with the DWARF unwinder.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>