dm-table: atomic writes support

Support stacking atomic write limits for DM devices.

All the pre-existing code in blk_stack_atomic_writes_limits() already takes
care of finding the aggregrate limits from the bottom devices.

Feature flag DM_TARGET_ATOMIC_WRITES is introduced so that atomic writes
can be enabled on personalities selectively. This is to ensure that atomic
writes are only enabled when verified to be working properly (for a
specific personality). In addition, it just may not make sense to enable
atomic writes on some personalities (so this flag also helps there).

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>