cifs: Reapply lost fix from commit 30b2b219

Reapply the fix from:

   30b2b219 ("cifs: do not include page data when checking signature")

that got lost in the iteratorisation of the cifs driver.

Fixes: d08089f6 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Acked-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Reported-by: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@cjr.nz>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Bharath S M <bharathsm@microsoft.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>