dm-crypt: avoid deadlock in mempools

This patch fixes a theoretical deadlock introduced in the previous patch.

The function crypt_alloc_buffer may be called concurrently. If we allocate
from the mempool concurrently, there is a possibility of deadlock.
For example, if we have mempool of 256 pages, two processes, each wanting 256,
pages allocate from the mempool concurrently, it may deadlock in a situation
where both processes have allocated 128 pages and the mempool is exhausted.

In order to avoid this scenarios, we allocate the pages under a mutex.

In order to not degrade performance with excessive locking, we try
non-blocking allocations without a mutex first and if it fails, we fallback
to a blocking allocation with a mutex.

CRs-fixed: 670391
Change-Id: I6c391dece4ba44fe0b2e9b75ea2b9235bf1b525b
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Patch-mainline: dm-devel @ 04/05/14, 14:07
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>