Use the correct root for shared jail when the source storage is also a jail

[icewind1991]

Fixesnextcloud/groupfolders#54

to test:

• enable groupfolders
• create a groupfolder Foo which can be accessed user user1
• as user1 create a folder Foo/sub and share it with user2
• as user2 upload a file in sub

Currently the file is uploaded to disk but not correctly added to the cache.

The root cause is sharing being smart an getting it's info efficiently from the db, which leads to some assumptions that were broken by groupfolders.

[codecov]

Codecov Report

Merging #7137 into master will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@ Coverage Diff @@## master #7137 +/- ##
============================================
+ Coverage 50.72% 50.74% +0.01% - Complexity 24478 24481 +3 
============================================
Files 1581 1581 Lines 93578 93589 +11 Branches 1359 1359 ============================================
+ Hits 47472 47494 +22 + Misses 46106 46095 -11

Impacted Files	Coverage Δ	Complexity Δ
apps/files_sharing/lib/Cache.php	87.93% <100%> (+0.89%)	22 <0> (+1)	⬆️
lib/private/Files/Storage/Wrapper/Jail.php	91.15% <71.42%> (+10.01%)	53 <2> (+2)	⬆️
lib/private/Security/CertificateManager.php	91.08% <0%> (-1%)	39% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.28% <0%> (-0.25%)	136% <0%> (ø)
lib/private/Server.php	83.41% <0%> (+0.12%)	125% <0%> (ø)	⬇️
lib/private/Files/Filesystem.php	69.01% <0%> (+0.78%)	117% <0%> (ø)	⬇️

[rullzer]

Makes sense!

[icewind1991]

@MorrisJobke@schiessle@nickvergessen please review

[MorrisJobke]

stable12 #7323