eCryptfs: cast page->index to loff_t instead of off_t

page->index should be cast to loff_t instead of off_t.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Reported-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>