dm-crypt: don't allocate pages for a partial request

This patch changes crypt_alloc_buffer so that it always allocates pages for
a full request.

This change enables further simplification and removing of one refcounts
in the next patches.

Note: the next patch is needed to fix a theoretical deadlock

CRs-fixed: 670391
Change-Id: I7bcadac8b3450976366c701fceb1fee7cb18df85
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[joonwoop@codeaurora.org: resolve trivial merge conflicts]
Patch-mainline: dm-devel @ 04/05/14, 14:07
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>