dm persistent data: fix return type of shadow_root()

shadow_root() truncates 64-bit dm_block_t into 32-bit int.  This is
not an issue in practice, since dm metadata as of v5.11 can only hold at
most 4161600 blocks (255 index entries * ~16k metadata blocks).

Nevertheless, this can confuse users debugging some specific data
corruption scenarios.  Also, DM_SM_METADATA_MAX_BLOCKS may be bumped in
the future, or persistent-data may find its use in other places.

Therefore, switch the return type of shadow_root from int to dm_block_t.

Signed-off-by: Jinoh Kang <jinoh.kang.kr@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/persistent-data/dm-btree-internal.h

@@ -100,7 +100,7 @@ struct dm_block *shadow_parent(struct shadow_spine *s);
 
 int shadow_has_parent(struct shadow_spine *s);
 
-int shadow_root(struct shadow_spine *s);
+dm_block_t shadow_root(struct shadow_spine *s);
 
 /*
  * Some inlines.

drivers/md/persistent-data/dm-btree-spine.c

@@ -235,7 +235,7 @@ int shadow_has_parent(struct shadow_spine *s)
 	return s->count >= 2;
 }
 
-int shadow_root(struct shadow_spine *s)
+dm_block_t shadow_root(struct shadow_spine *s)
 {
 	return s->root;
 }