Use file_source to find matching share for nested link shares

Fixes https://github.com/owncloud/core/issues/9242

Note that this if has been introduced by https://github.com/owncloud/core/pull/5632 for nested link shares.
To test:
1) Create a folder "test"
2) Share folder "test" with link
3) Enter folder "test"
4) Create a file "file.txt"
5) Share "file.txt" with link
6) Reload the page
7) Verify the the link for "test" and for "file.txt" work and point to the correct file
8) Unshare "file.txt"
9) See that "test" is still shared

Most importantly this is to distinguish the parent folder link share from the child file link share.

Please review @schiesbn @icewind1991 @MorrisJobke