mm: fix build warning for uninitialized value

mm: fix build warning for uninitialized value

do_wp_page() sets mmun_called if mmun_start and mmun_end were initialized
and, if so, may call mmu_notifier_invalidate_range_end() with these
values.  This doesn't prevent gcc from emitting a build warning though:

mm/memory.c: In function `do_wp_page':
mm/memory.c:2530: warning: `mmun_start' may be used uninitialized in this function
mm/memory.c:2531: warning: `mmun_end' may be used uninitialized in this function

It's much easier to initialize the variables to impossible values and do a
simple comparison to determine if they were initialized to remove the bool
entirely.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>