btrfs: close btrfs.FS handle after use

Despite being quite hard to provoke (< 10% in my testing), the btrfs
collector would occasionally leave stale FDs relating to btrfs
mountpoints, making the filesystems unable to be unmounted.

Fixes: #2772.

Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>

collector/btrfs_linux.go

@@ -135,6 +135,7 @@ func (c *btrfsCollector) getIoctlStats() (map[string]*btrfsIoctlFsStats, error)
 				"err", err)
 			continue
 		}
+		defer fs.Close()
 
 		fsInfo, err := fs.Info()
 		if err != nil {