eCryptfs: increment extent_offset once per loop interation

The extent_offset is getting incremented twice per loop iteration through any
given page.  It should only be getting incremented once.  This bug should only
impact hosts with >4K page sizes.

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