Fix sharing of ext storage mount points

When sharing an ext storage mount point, it will now use the name of the
mount point instead of an empty string for the target path.

apps/files_sharing/lib/share/file.php

@@ -33,10 +33,12 @@ class OC_Share_Backend_File implements OCP\Share_Backend_File_Dependent {
 	private $path;
 
 	public function isValidSource($itemSource, $uidOwner) {
-		$query = \OC_DB::prepare('SELECT `name` FROM `*PREFIX*filecache` WHERE `fileid` = ?');
-		$result = $query->execute(array($itemSource));
-		if ($row = $result->fetchRow()) {
-			$this->path = $row['name'];
+		$path = \OC\Files\Filesystem::getPath($itemSource);
+		if ($path) {
+			// FIXME: attributes should not be set here,
+			// keeping this pattern for now to avoid unexpected
+			// regressions
+			$this->path = basename($path);
 			return true;
 		}
 		return false;