dm writecache: fix smatch warning about invalid return from writecache_map

There's a smatch warning "inconsistent returns '&wc->lock'" in
dm-writecache. The reason for the warning is that writecache_map()
doesn't drop the lock on the impossible path.

Fix this warning by adding wc_unlock() after the BUG statement (so
that it will be compiled-away anyway).

Fixes: df699cc1 ("dm writecache: report invalid return from writecache_map helpers")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-writecache.c

@@ -1598,7 +1598,8 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 
 	default:
 		BUG();
-		return -1;
+		wc_unlock(wc);
+		return DM_MAPIO_KILL;
 	}
 }