btrfs: print a message when a missing device re-appears

There is a bug report that fstrim crashed, and that crash is eventually
pinned down to a missing device which re-appeared and screwed up callers
that only checks BTRFS_DEV_STATE_MISSING, but not
BTRFS_DEV_STATE_WRITEABLE nor device->bdev.

A missing device re-appearing can be very tricky, as for now it will
result in a device without WRITEABLE or MISSING flag, and still no bdev
pointer.

As the first step to enhance handling of such re-appearing missing
devices, add a dmesg output when a missing device re-appeared.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -963,6 +963,11 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 					  devid, btrfs_dev_name(device),
 					  path, current->comm,
 					  task_pid_nr(current));
+		} else {
+			btrfs_info(NULL,
+	"missing devid %llu re-appeared at %s scanned by %s (%d)",
+				   devid, path, current->comm,
+				   task_pid_nr(current));
 		}
 
 		name = kstrdup(path, GFP_NOFS);