page_mapping(): add ifdef around reference to swapper_space

This fixes the superh build when the pageflags patches are applied.

But it shouldn't unless it's a gcc bug.

Cc: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>