dm-crypt: offload writes to thread

Submitting write bios directly in the encryption thread caused serious
performance degradation. On multiprocessor machine encryption requests
finish in a different order than they were submitted in. Consequently, write
requests would be submitted in a different order and it could cause severe
performance degradation.

This patch moves submitting write requests to a separate thread so that
the requests can be sorted before submitting.

Sorting is implemented in the next patch.

Note: it is required that a previous patch "dm-crypt: don't allocate pages
for a partial request." is applied before applying this patch. Without
that, this patch could introduce a crash.

CRs-fixed: 670391
Change-Id: I886ed2da0ff174d3539ea18e27170d7fd1062680
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Patch-mainline: dm-devel @ 04/05/14, 14:08
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>