sh: Shut up compiler warnings in __do_page_fault().

GCC doesn't seem to be able to figure this one out for
itself, so just shut it up..

  CC      arch/sh/mm/fault.o
arch/sh/mm/fault.c: In function '__do_page_fault':
arch/sh/mm/fault.c:288: warning: 'ptl' may be used uninitialized in this function

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