core/swap: support "systemctl clean" for swap units

src/basic/unit-def.c

@@ -219,7 +219,8 @@ static const char* const swap_state_table[_SWAP_STATE_MAX] = {
         [SWAP_DEACTIVATING] = "deactivating",
         [SWAP_DEACTIVATING_SIGTERM] = "deactivating-sigterm",
         [SWAP_DEACTIVATING_SIGKILL] = "deactivating-sigkill",
-        [SWAP_FAILED] = "failed"
+        [SWAP_FAILED] = "failed",
+        [SWAP_CLEANING] = "cleaning",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(swap_state, SwapState);

src/basic/unit-def.h

@@ -162,6 +162,7 @@ typedef enum SwapState {
         SWAP_DEACTIVATING_SIGTERM,
         SWAP_DEACTIVATING_SIGKILL,
         SWAP_FAILED,
+        SWAP_CLEANING,
         _SWAP_STATE_MAX,
         _SWAP_STATE_INVALID = -1
 } SwapState;