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

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

Change crypt_alloc_buffer so that it only ever allocates pages for a
full request.

This change is a prerequisite for the commit "dm crypt: offload writes
to thread".  Which implies this change is effectively required for the
upcoming cpu parallelization changes.

But this change simplifies the dm-crypt code at the expense of reduced
throughput in low memory conditions (where allocation for a partial
request is most useful).

This change also enables the removal of the io_pending refcount.

Note: the next commit ("dm-crypt: avoid deadlock in mempools") is needed
to fix a theoretical deadlock.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>