[POWERPC] Make rheap safe for spinlocks

The rheap allocation function, rh_alloc, could call kmalloc with GFP_KERNEL.
This can sleep, which means you couldn't hold a spinlock while called rh_alloc.
Change all kmalloc calls to use GFP_ATOMIC so that it won't sleep.  This is
safe because only small blocks are allocated.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>