dm crypt: avoid deadlock in mempools

dm crypt: avoid deadlock in mempools

Fix a theoretical deadlock introduced in the previous commit ("dm crypt:
don't allocate pages for a partial request").

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 such a scenario, 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.

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