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>

include/linux/device-mapper.h

@@ -299,6 +299,9 @@ struct target_type {
 #define dm_target_supports_mixed_zoned_model(type) (false)
 #endif
 
+#define DM_TARGET_ATOMIC_WRITES		0x00000400
+#define dm_target_supports_atomic_writes(type) ((type)->features & DM_TARGET_ATOMIC_WRITES)
+
 struct dm_target {
 	struct dm_table *table;
 	struct target_type *type;