cifs: Convert cifs to new aops.

This patch is based on the one originally posted by Nick Piggin. His
patch was very close, but had a couple of small bugs. Nick's original
comments follow:

This is another relatively naive conversion. Always do the read upfront
when the page is not uptodate (unless we're in the writethrough path).

Fix an uninitialized data exposure where SetPageUptodate was called
before the page was uptodate.

SetPageUptodate and switch to writeback mode in the case that the full
page was dirtied.
Acked-by: Shaggy <shaggy@austin.ibm.com>
Acked-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>