From: Miquel van Smoorenburg <miquels@cistron.nl>
      Joe Thornber <thornber@redhat.com>

This implements the queue congestion callout for DM stacks.  To make
bdi_read/write_congested() return correct information.

- md->lock protects all fields in md _except_ md->map
- md->map_lock protects md->map
- Anyone who wants to read md->map should use dm_get_table() which
  increments the tables reference count.

This means the spin lock is now only held for the duration of a
reference count increment.

Udpate:

dm.c: protect md->map with a rw spin lock rather than the md->lock
semaphore.  Also ensure that everyone accesses md->map through
dm_get_table(), rather than directly.