Impact: fix bootup crash on xen guests

SLAB is not yet up, with earlyprintk it is giving me an Oops in __kmalloc.

Replace call to kmalloc() with alloc_bootmem().
Reported-by: NChristophe Saout <christophe@saout.de>
Signed-off-by: NMike Travis <travis@sgi.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>