sgi-xp: add 'jiffies' to reserved page's timestamp name

Rename XPC's reserved page's timestamp member to reflect the units of time
involved.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>