Add dm-writecache support

dm-writecache is used like dm-cache with a standard LV
as the cache.

$ lvcreate -n main -L 128M -an foo /dev/loop0

$ lvcreate -n fast -L 32M -an foo /dev/pmem0

$ lvconvert --type writecache --cachepool fast foo/main

$ lvs -a foo -o+devices
  LV            VG  Attr       LSize   Origin        Devices
  [fast]        foo -wi-------  32.00m               /dev/pmem0(0)
  main          foo Cwi------- 128.00m [main_wcorig] main_wcorig(0)
  [main_wcorig] foo -wi------- 128.00m               /dev/loop0(0)

$ lvchange -ay foo/main

$ dmsetup table
foo-main_wcorig: 0 262144 linear 7:0 2048
foo-main: 0 262144 writecache p 253:4 253:3 4096 0
foo-fast: 0 65536 linear 259:0 2048

$ lvchange -an foo/main

$ lvconvert --splitcache foo/main

$ lvs -a foo -o+devices
  LV   VG  Attr       LSize   Devices
  fast foo -wi-------  32.00m /dev/pmem0(0)
  main foo -wi------- 128.00m /dev/loop0(0)

1 files changed, 2 insertions, 1 deletions

diff --git a/tools/tools.h b/tools/tools.h
index 55d486b05..5e0cd3046 100644
--- a/tools/tools.h
+++ b/tools/tools.h
@@ -248,11 +248,12 @@ int lvconvert_start_poll_cmd(struct cmd_context *cmd, int argc, char **argv);
 
 int lvconvert_to_pool_cmd(struct cmd_context *cmd, int argc, char **argv);
 int lvconvert_to_cache_vol_cmd(struct cmd_context *cmd, int argc, char **argv);
+int lvconvert_to_writecache_vol_cmd(struct cmd_context *cmd, int argc, char **argv);
 int lvconvert_to_thin_with_external_cmd(struct cmd_context *cmd, int argc, char **argv);
 int lvconvert_swap_pool_metadata_cmd(struct cmd_context *cmd, int argc, char **argv);
 int lvconvert_to_pool_or_swap_metadata_cmd(struct cmd_context *cmd, int argc, char **argv);
 int lvconvert_merge_thin_cmd(struct cmd_context *cmd, int argc, char **argv);
-int lvconvert_split_cachepool_cmd(struct cmd_context *cmd, int argc, char **argv);
+int lvconvert_split_cache_cmd(struct cmd_context *cmd, int argc, char **argv);
 
 int lvconvert_raid_types_cmd(struct cmd_context * cmd, int argc, char **argv);
 int lvconvert_split_mirror_images_cmd(struct cmd_context * cmd, int argc, char **argv);