ecryptfs: fix fsx data corruption problems

ecryptfs in 2.6.24-rc3 wasn't surviving fsx for me at all, dying after 4
ops.  Generally, encountering problems with stale data and improperly
zeroed pages.  An extending truncate + write for example would expose stale
data.

With the changes below I got to a million ops and beyond with all mmap ops
disabled - mmap still needs work.  (A version of this patch on a RHEL5
kernel ran for over 110 million fsx ops)

I added a few comments as well, to the best of my understanding
as I read through the code.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-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>