dm-ebs: fix prefetch-vs-suspend race

There's a possible race condition in dm-ebs - dm bufio prefetch may be in
progress while the device is suspended. Fix this by calling
dm_bufio_client_reset in the postsuspend hook.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org