backup: convert reject funcs to use FS interface

Depending on parameters the paths in a snapshot do not directly
correspond to real paths on the filesystem. Therefore, reject funcs must
use the FS interface to work correctly.

internal/fs/fs_local.go

@@ -46,6 +46,12 @@ func (fs Local) Lstat(name string) (os.FileInfo, error) {
 	return os.Lstat(fixpath(name))
 }
 
+// DeviceID extracts the DeviceID from the given FileInfo. If the fs does
+// not support a DeviceID, it returns an error instead
+func (fs Local) DeviceID(fi os.FileInfo) (deviceID uint64, err error) {
+	return DeviceID(fi)
+}
+
 // Join joins any number of path elements into a single path, adding a
 // Separator if necessary. Join calls Clean on the result; in particular, all
 // empty strings are ignored. On Windows, the result is a UNC path if and only